You are so intimately familiar with certain spells that you don’t need a spellbook to prepare them anymore.
Prerequisite: Wizard level 1st.
Benefit: 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.
Normal: Without this feat, you must use a spellbook to prepare all your spells, except read magic.
Source: Player’s Handbook (version 3.5), page 100.