When Spellbook is cast it grants a mage +1 bonus to skill on each spell that is inscribe into the book. The main downside of this is "the book must remain in the mage’s possession to gain this advantage; if he loses it or it is stolen, he not only loses the +1 bonus, but also gets a -1 penalty to those spells until the book is retrieved."
All the spells in the book function as Scrolls but if anyone but the mage who cast Spellbook uses the spell as a Scroll the page goes blank and the maker is at -3 to cast that spell until it is reinscribed in the book.
Worst if anyone, even the who cast Spellbook, removes a page the mage forgets the spell entirely and must relearn the spell if it is not returned to the book.