You can draw on the power of two spellbooks at once.
Prerequisite: Two spellbooks, one metamagic feat
Benefit: You can prepare a single spell slot with two different spells, which must be prepared from two different spellbooks. The total level of these two spells must equal the level of the spell slot used. You can cast either one of these spells from that slot, but not both. Once one of those two spells is cast from the spell slot, it is expended until the next time you prepare spells.
Some spellcasters are all about versatility, and this feat can make you a master of such magic.