Hello
I had the same question looking through the CRB earlier. Reading through the wizard description, it kept referring to "his Spell book". I had found the wizard spellbook in the equipment section. It says there that it has 100 pages and that each spell takes up one page per spell level (0 lvl spells take 1 page).
I couldn't find any mention of the number of spells they are able to put in per lvl, if any spells can be removed once written in, or anything. I had the idea to let them start with something similar to what a sorcerer has, and then making a research requirement time to learn a new spell. higher lvl spells needing more time.
OOooo, and spells written with ink in the book can't be removed. maybe have them be required to craft a new book from scratch if they wish the spells to change. Unless they or someone else has the erase spell. mmmmm, idk. im just learning.
I am actually very new to this game. i have only played 2 times, and have been rather confused most the time lol. Im starting to get things, but i think its just my group is almost all new to the game. not many of us have ever played anything like this.
My siblings have taken an interest in the game as well. so i plan on using them as guinea-pigs. They wont really care if im fumbling through as a GM. Im glad theres this site to reference for ideas.