Spellbooks are for Wizards copying spells only. Other classes can't actually profit from it. Even the Trickster has no proper use for it (see here ). As Szega mentioned, the party could get some benefit from it through Ritual Casting. They could also get a larger benefit through multiclassing (into Wizard - and I should note that MCing into Wizard is usually not optimal), but both these solutions depend on the party, not on you.
You can change it, though
You mentioned this is just a game with your friends, so you have a few options:
- Change the reward. Your original question asked what would be a good reward - and that's hard to tell. But you are in a game with friends, so you're completely allowed to. I would check items given in other published adventures (there is this nice sheet listing them all by adventure) in a similar level, so you can be sure your power level is fine.
- A suggestion from Bloodcinder: give them spell scrolls (that they can use) instead. Note that while spell scrolls are single-use, they don't spend material components or spell slots - so their one use is stronger than simply having that spell in your list. How many spell scrolls and what spells is, again, up to you.
- Change the reward without actually changing it - introduce a new, friendly Wizard that is interested in that book, and he's willing to trade something that has little use for him, but has more use for your party.
- Use it as plot - similar to the point above, you can introduce the Wizard and, instead of him giving other magic items as a reward, he can offer services, information or a new quest for the party. If you are interested in playing the adventure as it is, this one is the worst one, but if you are more free about it, this might be the funniest.
I rarely have parties that play Wizard, as I usually DM for new players and they find Wizards too complicated (pff). Nonetheless, I have used lots of Evil Wizard in my campaigns, and they obviously had their beloved spellbooks, which would be a part of the loot. I have applied my third and fourth bullets quite frequently, and usually they led to fun side-quests and interactions. Properly adjusting the reward can be challenging, but again: you are in a table with friends, don't worry too much - it's fine if they get an item a little more overpowered than it should, you can just make the encounters a little harder.