There are two types of spellbooks: Traveling and Standard.
Standard spellbooks are made to handle the rigors of time and sitting in a library. They have up to 100 pages, and weigh about 20lbs. Cost to buy 50GP per page.
Traveling spellbooks are made to handle travel better, and are more compact. They have up to 50 pages and weigh about 7lbs. Cost to buy 100GP per page.
Every spell takes up a number of pages in a wizard’s spellbook, and the number varies from wizard to wizard as they each individualize their way of depicting the spell and how it is easiest to memorize. The number of pages a given spell takes up is: 1d6-1 + spell level