I need some suggestions! One of my players wants to visit the Headmaster of the SandPoint Turandarok Academy and copy spells from his spell book. They have a good history together since the PC was raised in the orphanage. The headmaster is a Wizard 4/Rogue 2, how would I go about determining how many and what spells they are likely to have in his spell book?
Why would being raised in the orphanage necessarily indicate a 'good history' between the PC and the headmaster wizard? Perhaps the headmaster is indifferent to the majority of his students. Perhaps this particular student rubbed him the wrong way at some time.
Even if the headmaster had a decent opinion of the PC, why would he just freely give away spells in his spellbook? Spells are a commodity, they can be bought and sold like any other commodity. Maybe he gives the PC a break, but still charges him for the spells. After all, it is unlikely that the headmaster got the spells for free himself.
Most wizards in the games I play (PCs and NPCs) are very, very protective of their spellbooks. It is unlikely any of my PCs would just hand over a spellbook to any but the most trusted of close friends.
Having gone through that, if you still want to give the PC this opportunity to grab some free spells, the thing to do would be to figure out what sort of spells a wizard 4/rogue 2 who is headmaster of an academy would have in their spellbook. In my opinion those would mostly be spells that would help a rogue to sneak or spells that would make running a school easier. It would be very unlikely that the headmaster would have powerful offensive spells since that's just not what they do. Divination, enchantment, illusion would be the most likely spells I would think.
Having said all that, I have to say that this smacks to me of a player trying to exploit a "role playing" relationship to gain an in-game advantage.
for starters: ll cantrips, 5+INT level one spells, 4 level two spells. He gets those just for being a 4th level wizard. Safe to assume as an academy headmaster, he has access and opportunity to copy other spells into his spellbook, so add more as you like, but don't have less than the class ability gives him automatically.
He's also a rogue, so I would assume he favors stealthy rogue-ish spells like Invisibility, Silence, Knock, etc., as well as anti-rogue stuff like See Invisible. He also runs an academy, so spells that provide information or help him run an academy seem sensible, such as Comprehend Languages or Identify.
After that, maybe consider his previous career. Did he adventure as a wizard? If so, then he'd have basic adventuring spells (which is probably the most useful for an adventuring wizard who wants to buy spells from this guy).
Finally, add whatever you want. Me, I often know what a player is looking for. Maybe other players have advised this player to go find Glitterdust or Create Pit or something and I've heard their conversations, so I would put at least one or two such spells so the player will be happy.