Spell Mastery( Player's Handbook v.3.5, p. 100)
You are so intimately familiar with certain spells that you don't need a spellbook to prepare them anymore.
Wizard level 1,
Each time you take this feat, choose a number of spells equal to your Intelligence modifier that you already know. From that point on, you can prepare these spells without referring to a spellbook.
Without this feat, you must use a spellbook to prepare all your spells, except read magic.