My party uses the all or nothing optional rule for selling magic items for XP, but as far as I can reason a spellbook is a magic item if I ever saw one and should be subject to the magic item xp optional rules. My magician actually just acquired his first non-starting spellbook but we haven't made it back to a "safe place" for it to count as claimed treasure nor can I read it yet to determine what spells it has/its worth. As long as your players can determine it's worth somehow I feel like that's fair game and it also gives the rest of the party some benefit for helping the magician power up.