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.