If a Wizard finds the spellbook of the evil Wizard they discovered, how does the process of finding out which spells are in the spellbook work? (Assume the evil Wizard didn't simply write the spells' names in common at the top of the pages or create an index.)
- Does the Wizard have to spend 2 hours per spell level "deciphering
the unique system of notation" to be able to tell that the spell
they've been studying is detect magic? Or can they tell at a glance
that it's detect magic before spending the 2 hours? - Does it require the 50gp per spell level cost just to figure out what
the spell is, or do they get to choose whether to spend it after
figuring out what spell it is, as part of deciding whether to copy
the spell into their spellbook?
From the 5e Basic Rules, Chapter 3, pg. 31 (emphasis mine):
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.
Best Answer
Rules As Written: It's unclear
Rules As Intended: You likely know which spell it is
5th edition has tried to stay away from "GOTCHA!" moments, so it feels against the spirit of the game that they would make you spend time and money, only to find out that you don't want to copy "Pete's Portable Outhouse" into your spellbook, or whatever.
There are various modules wherein players find a spellbook "with X,Y, and Z spells" and no mentions of "after the wizard deciphers part of the spellbook, they realize that these are the spells in the book."
Additionally, that you could potentially start trying to copy down a spell that you can't prepare (a level 2 spell, while you're still level 1) would potentially be possible if you didn't know what the spell was. Would that mean that you've wasted the 2 hours per level and 50 GP per level up to what you could potentially prepare? Would the DM just tell you "You try to decipher the spell and fail."? Neither of those seem to be keeping with game intent.
Rules As Fun (and Rules As Common Sense, likely): You definitely know what the spells in the book are
It is neither fun, nor interesting, nor interactive in any way to make players decipher what the spell is before they can decide whether or not they want it.
Just imagine being a first-level wizard and your DM telling you "You find a dusty tome in the abandoned wizard's tower. You have to spend at least 50 gold pieces each to find out what the ten spells within can do."
You'd sell the book for what you could and walk away (or at least I would).
...
Another interaction occurs to me. If you want to pay to copy a spell out of a wizard's book, you wouldn't actually know what spell you were copying until you were done. While hilarious once, it would be a huge jerk move on both the side of the wizard and the DM allowing it.