Multiclassing mid-dungeon into Wizard. Where's my spellbook? When do I prepare my spells?

Question

In one of my campaigns I have a character that just leveled up. I've been planning for a while to multi-class into Wizard, but never really had the character RP prepare for the event, since I didn't expect to level up mid-dungeon (we're using milestone XP). I always figured we'd end the dungeon run, head to the nearest inn, and I'd be able to level up and prepare there - buy the spell book in town, and so on.

We're nowhere near done with the dungeon, so I'm not going to buy a spellbook anytime soon. I'm looking for creative options to get around this hurdle without having the party wait for this guy over here to prepare his book. Thus, my question is two fold:

• Can I immediately just use the 3 cantrips I've selected?
• What would be a reasonable approach to scrap together a prepared spellbook, having no prior scrolls and no spells?

So far I've written a short story for the DM explaining that my character "secretly bought an empty notebook a while back (subtract whatever GP is appropriate)" and that he's been "observing the other spell casters in the party" and wanting to get into magic, but "it wasn't until the last battle it all clicked for him."

Not sure if that's going to fly - and honestly I'm okay if it doesn't. I want to be realistic within reason, but I also feel like there's a compromise to keep things fun. Could my character overnight just suddenly unlock a prepared spellbook? Would it be more reasonable to craft one, and then during each rest slowly build up the prepared spells?

Answer 1

You can multiclass and will have spells and spellbook

Your spellbook is part of your Spellcasting ability. You do not need to buy one when you level into your first level of wizard. How and if you narratively justify that is up to you and your DM.

Your spellbook can be anything, it does not need to be a proper spellbook. It can be scrap notes you made adventuring. Page 114 PHB

Your spellbook is a unique compilation of spells, with its own decorative flourishes and margin notes. It might be a plain, functional leather volume that you received as a gift from your master, a finely bound gilt-edged tome you found in an ancient library, or even a loose collection of notes scrounged together after you lost your previous spellbook in a mishap.

You will immedately have access to your cantrips. Also page 114 PHB:

Cantrips
At 1st level, you know three cantrips of your choice from the wizard spell list.

You need to prepare your leveled spells, which you typically can only do after a long rest. Given that you just achieved a milestone, that might be a natural point for a rest/break anyways? Still page 114:

You prepare the list of wizard spells that are available for you to cast. (…) You can change your list of prepared spells when you finish a long rest. Preparing a new list of wizard spells requires time spent studying your spellbook and memorizing the incantations and gestures you must make to cast the spell: at least 1 minute per spell level for each spell on your list."

All of this assume that your DM is OK with multiclassing, which like feats is optional, but is typically allowed and used.

Answer 2

I know it's too late for you now, but for future readers:

If you plan on multiclassing, you should plan

To be a wizard, you need to experiment:

They learn new spells as they experiment and grow in experience.

The spells that you add to your spellbook as you gain levels reflect the arcane research you conduct on your own, as well as intellectual breakthroughs you have had about the nature of the multiverse.

It would only make sense that even before you actually get the first level of the Wizard class you already have started experimenting on the spells you are going to get.

This learning and experimentation is the time when you should be able to assemble your spell component pouch or find an item that'll work as your focus, and fill a book with notes to make it a spellbook.

Most of DMs I know (that allows multiclassing) would be glad to know in advance your multiclassing plans and help you to go smoothly and without retcons, by making sure there's always a stack of papers somewhere, an almost empty journal, a weird gem that if mounted on just right would allow your cane to channel arcane energy, and so on.

Your DM is there to cooperate. To write the story together. Use that. Help them to help you!