Can the Book of Shadows from a Warlock pact double as a Wizard's spell book if the character has multiclassed? If the Book of Shadows was lost and the character summoned a new one, would the transcribed spells return as well?
There is nothing mechanically preventing it, so talk to your DM about it
A wizard's chooses their spellbook and it can be almost anything - even a bag of rocks
Xanathar's Guide to Everything, goes into even more detail about a Wizard's spellbook and how they definitely have a great amount of flexibility when choosing what to use.
Your wizard character’s most prized possession — your spellbook — might be an innocuous-looking volume whose covers show no hint of what’s inside. Or you might display some flair, as many wizards do, by carrying a spellbook of an unusual sort. If you don’t own such an item already, one of your goals might be to find a spellbook that sets you apart by its appearance or its means of manufacture.
It includes examples of spellbooks of differing designs and compositions such as:
Small stones inscribed with spells and kept in a cloth bag
A tome with pages that are thin sheets of metal, spells etched into them with acid
So, a wizard can literally inscribe their spells on rocks if they want to. There's no indication here if this is actually any restriction on what is allowed besides that you be able to write/inscribe on it. If anything, it seems they are encouraging players to find creative things to use as spellbooks.
Book of Shadows easily fits the description of a standard spellbook
Pact of the Tome. Your Book of Shadows might be a fine, gilt-edged tome with spells of enchantment and illusion, gifted to you by the lordly Archfey. It could be a weighty tome bound in demon hide studded with iron, holding spells of conjuration and a wealth of forbidden lore about the sinister regions of the cosmos, a gift of the Fiend. Or it could be the tattered diary of a lunatic driven mad by contact with the Great Old One, holding scraps of spells that only your own burgeoning insanity allows you to understand and cast.
Narratively, many of these things sounds like they could be describing a wizard's spellbook. In fact, even more like a typical spellbook than many of the options listed above that are allowed for actual spellbooks (rocks eg).
Given the similarity, your DM might reasonably rule that you can use a Book of Shadows as a spellbook
The fact that they are narratively very similar and that there is nothing in either class or in the mechanics of the tomes themselves that would preclude such a use says to me that it is likely able to be used without much worry in a game.
I actually cannot think of a way I would reasonably be able to say to my player that they aren't able to write spells in their Book of Shadows but rocks are totally ok. You can write in it and it seems that is enough.
Of course, this is an unusual use and your DM must be the one to make the final call on whether they agree with it.
Besides a potential (significant) advantage against losing the Spellbook of Shadows, there really isn't much in the way of mechanical advantage or disadvantage to be had here. And because this advantage comes from a player's choice in class, I am fine to give it to them personally.
Replacing your Book of Shadows/Spellbook (Wizard spells probably will not regenerate)
If you lose your Book of Shadows, you can perform a 1-hour ceremony to receive a replacement from your patron. This ceremony can be performed during a short or long rest, and it destroys the previous book. The book turns to ash when you die.
The rules don't say here whether any modifications you made to the book (eg adding wizard spells to it) are preserved when the new book is summoned. This will have to be a DM decision on how they want this to work (if they allowed it in the first place).
If the DM says that the inscribed spells are retained, then you have a spellbook that is impossible to lose.
However, it seems much more reasonable to rule that they do not. The powers that give the Book of Shadows likely have no interest in standard wizard spells and may actually dislike them (thanks @ShadoCat).
If the DM rules that the inscribed spells are not retained, then you have to follow the standard wizard rules for inscribing spells. It never hurts to have a backup "standard" wizard spellbook, just in case.
Note that just because you are putting wizard spells in your book of secrets does not make them warlock spells.
The mechanics of which spells are considered class spells are outlined in each class and in the rules elsewhere. Using a Book of Shadows as a spellbook will not change these. This is essentially just a retheme of the case in which you would have two separate books with very little mechanical difference.
Yes. A Warlock's Book of Shadows can also be used as a Wizard's spellbook.
The most important thing about a Wizard's spellbook is that it is their personal spellbook, where they have described in their own manner and notation how to cast a spell. They may only prepare Wizard spells from their own spellbook. There is a section in the Wizard class writeup (PHB 114) which gives an open-ended description for how the spellbook might physically appear.
Nothing in the description of the Pact of the Tome (PHB 108) feature for Warlocks contradicts the loose rules on spellbooks for Wizards. It should be possible for the spellbook given by the Warlock Patron to also be usable for Wizard spells.
The description for spellbook under Adventuring Gear (PHB 150) is an example of a typical spellbook which might be easily purchased for 50 gp and weighs 3 lbs. It happens to have 100 blank vellum pages when bought.
The grimoire given to the Warlock by her Patron might indeed be (physically) the same as a basic spellbook that you can buy from a merchant, but it should not have to be. Likewise, there does not appear to be a hard rule on the number of pages that can be in a spellbook or against other forms or recording spells.
(This answer expands on Slagmoth's comment pointing out the equipment description as a typical example, not a hard rule.)
No. They are two distinct items.
A spellbook is a specific item in the Player's Handbook; not just any book will do. The magic of the Book of Shadows does not constitute the requirements for the spellbook item
More specifically, a spellbook is made with vellum pages whereas the grimoire granted by your pact does not ensure this.
a spellbook is a leather-bound tome with 100 blank vellum pages
Yes and No.
There is no reason why a warlock/wizard couldn't write his spells into the Book of Shadows and use that portion of it as his spellbook. It will have to be tracked separately, i.e. any spells written in it as warlock spells (if you have the Book of Ancient Secrets invocation) will not be able to be prepared as wizard spells.
What is in a newly summoned Book of Shadows replacing a lost one is a DM call. Certainly the three cantrips, yes. There is debate as to whether the replacement book even comes with all the rituals added to it if you have the BoAS; I rule that it does, that the added rituals become part of the book provided (and replaced if needed) by the patron. I would rule that the replacement book would not include the wizard spellbook portion (or, for that matter, any poems, recipes, diary entries, or lunatic ravings) unless prior arrangement of that is made with the patron (and the DM).
RAW, you can not use it as a Wizard's Spellbook
A Wizard's Spellbook requires (PHB, 114) limits a spellbook to contain only Wizard spells.
When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a level for which you have spell slots and if you can spare the time to decipher and copy it.
This is a hard limit that doesn't allow the possibility of inscribing spells from other lists in it without having to homebrew a solution for a combined Wizard/Warlock book and what happens on recall.
The Warlock's Book of Shadows
The Pact of the Tome (PHB, 108) provides this book at third level, with the following capability:
When you gain this feature, choose three cantrips from any class’s spell list. While the book is on your person, you can cast those cantrips at will. They don’t count against your number of cantrips known.
Unless you take the Book of Ancient Secrets Invocation (PHB, 110), there isn't even any mechanic allowing the Warlock to add any spells into the book. And even with that specific Invocation, they can only add Ritual spells.
But a book is a book, right?
One would think that because the Book of Shadows is a book, it could double as a Wizard's Spellbook for containing spells. There are, however, a couple of problems with that assumption:
- Warlock's can't add new spells to the Book unless they take the Invocation for adding Ritual Spells.
- Wizard's can only have Wizard Spells in their spellbook
Bypassing the recreation of a Wizard's Spellbook
The Book of Shadows allows replacement (PHB, 108)
If you lose your Book of Shadows, you can perform a 1-hour ceremony to receive a replacement from your patron.
A wizard is very dependent on their spellbook, and the mechanics to replace it are not simple - especially for anything that wasn't prepared.
If you lose your spellbook, you can use the same procedure to transcribe the spells that you have prepared into a new spellbook. Filling out the remainder of your spellbook requires you to find new spells to do so, as normal. For this reason, many wizards keep backup spellbooks in a safe place.
By putting spells in a book that can be summoned in it's entirety back after loss or destruction, you are significantly increasing the power of a Wizard by removing that obstacle. This is not a minor obstacle, either. Wizards need to spend time and money in order to make copies and keep them safe. By removing that, you are eliminating a time and money consuming mechanic and ignoring several of the requirements for both Wizards and Warlocks.
A workaround the removes the concern of what happens on recalling the book after it's destruction
Rather than trying to bypass the existing rules around the Book of Shadows and Wizard Spellbooks, just have the Spellbook stuffed with loose sheets of paper with the Wizard's spells on them. It's effectively the same thing as writing them in the book, but it removes any chance of player/DM arguments on whether or not the Wizard spells return.
In addition, it does allow the Warlock/Wizard to inscribe Rituals into the book (if they took that invocation) that would return on recall - while the loose leaf sheets of Wizard spells would not.
A reason to allow
Rubiksmoose's answer is very well thought out, and as long as the Wizard spells do not reappear when the Book of Shadows is replaced, then there is little risk. It is just very important that both players and DMs understand that those spells will be lost upon destruction, loss, or replacement of an existing Book of Shadows.