Can the wizard in our group prepare a spell from the spellbook of an NPC wizard?

Question

Whether the players killed a mage and got his spellbook(s) or they met with a nice wizard who let him or her learn a spell from his spellbook, can a player prepare a spell from a spellbook that isn't his or hers?

I'm asking because the PHB p. 114 says:

[...] 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.

• emphasis mine

Further, the block on the left as a Copying a Spell into the Book ("the" Book as in "your" Book, since it appears in the block titled Your Spellbook), which explains how you can copy a spell from one book to another, assuming the book is from a stranger, it cost more and takes more time. It actually says:

[...] deciphering the unique system of notation used by the system of notation of the wizard who wrote it. [...]

It continues saying that you have to learn the various parts of the spell (verbose, material, gesture.)

This makes it sounds like a wizard would have a really hard time to prepare a spell from another's spellbook. Is that the case?

Answer 1

This makes it sounds like a wizard would have a really hard time to prepare a spell from another's spellbook. Is that the case?

That is correct.

As you quoted, each wizard's spellbook notation is unique, therefore you can't prepare spells from someone else's book.

You must first copy the spell into your own book (deciphering the notation in the process), after which you can prepare it as normal.

Answer 2

This is how Wizards prepare spells (PHB p. 114):

You prepare the list of wizard spells that are available for you to cast. To do so, choose a number of wizard spells from your spellbook ...

You can only prepare spells that are in your spellbook. You can't crib off someone else's notes!

This is how you add spells to your spellbook (PHB p. 114):

Copying a Spell into the Book. 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.

Copying a spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation.

For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it. Once you have spent this time and money, you can prepare the spell just like your other spells.

"This makes it sounds like a wizard would have a really hard time to learn a spell from another's spellbook. Is that the case?"

This is the case: it takes 2 hours and costs 50gp per spell level - 2 hours and 50gp for a 1st level spell, 18 hours and 450gp for a 9th level spell. Personally, 2 days work and 450gp to learn how to cast a Wish spell doesn't seem like a "hard time" to me but everyone has their own standards of what constitutes a "hard time".

Answer 3

The idea behind the spellbook is that you write down movements, chants and hints how to gather, focus and bend the energy within you and your surroundings to your own will. Each caster will have his own way of accomplishing these things. The idea to study someone else's spellbook, reading and experimenting over it - is to exactly translating their ways of controlling energy into your own known techniques.

e.g. One wizard may have problems with the correct order of incantations he has to recite for a specific cast. Thus in his spellbook he usess jingles and clues to guide him remembering those spells. But on the other side he is really good at gathering the energy with his gestic and mimic and only wrote down some abbreviations to hint the correct movements.

If some other wizard would read the 1st casters spellbook, the 2nd has to invest time and money for some ink + experimenting resources to translate and decode the other wizards style of magic. After that time and payment he manages to write down the spell in a way that guides himself through the spell and allows him to prepare it.

Answer 4

If a Wizard gives you a filled spellbook, it is yours.
If you find a filled spellbook as loot, it is yours.
If you kill an evil Wizard and take his spellbook, it is yours.

Loot found belongs to the player characters; that is one of the most basic rules in D&D unless the DM is being a jerk. So I see no reason why you can’t cast spells from such spellbook. If you're smart enough to decipher another’s book to copy it down, then you are smart enough to cast the spell.