In extremis the DM may allow a magic-user to cast a spell directly from any sort of spell book just as if the book were a scroll. The book must be of appropriate sort so that the spell matches the profession of the caster, i.e. magic-user spell, magic-user spell book. The caster must have read the particular spell. The caster must be able to know and use the spell in question. (Note that in this regard, reading directly from a spell book differs from the use of scroll spells!) Direct casting of a spell from a spell book automatically destroys that spell. There is also a 1% chance per level of the spell that the spells immediately preceding and following the spell cast will likewise be destroyed. There is an additional 1% chance that the casting of a spell directly from a spell book will destroy the entire book. A Permanency spell, for instance, would not prevent a spell from “disappearing” when cast in this manner; even though writing might remain on the page, that
writing will no longer be magical in nature. These strictures apply whether a spell caster is using his or her personal book or the book of another. Read Magic is required to read another mage’s spell book, and a magic-user can learn a spell by reading it in another’s book. This learning process requires 2-8 hours of study per spell level, after which time the spell is learned and thereby immediately usable by the M-U doing the studying.
Dragon Magazine 62, p15