By holding a Thief spellbook (see help 'memorize spell') an invoker,
transmuter or necromancer may scribe a spell he/she knows into the book.
The book will contain this spell forever, and a thief may use it to
memorize the spell.
Note: Not all spells may be learned by thieves, even though you can
scribe them.
See also: help 'memorizing spell'