Book of Infinite Spells is a book which can be equipped in the item slot by every class. It allows to cast one of the ten available spells once a day. It can also be used to turn a page, which changes the spell the book can be used to cast, but after you turn to the last page you can not turn the page any more. It can be gained from Perth the Adept in Brynnlaw who guards the Asylum Wardstone as well. This item appears only in Baldur's Gate II: Shadows of Amn.
The spell appears to be random, and so through judicious use of saving and loading, the spell the player prefers may be selected.
This ancient tome creaks as you open it. Each page of the Book of Infinite Spells contains a spell described in layman's terms, easy enough for a thief, a cleric or even a fighter to understand and cast it. Each spell on a page can be cast once per day.
A second function of the book is to turn to a different page of the book. This allows for the possibility of getting a better spell. However, once the page has been turned the previous spell is lost forever, unless it happens to appear again.
Be warned. Once you turn to the last page of the book, you are stuck with that spell.