You can actually use captured spellbooks as a spare copy for spells you have already learned or decide to learn. Prepare a spell from a borrowed spellbook is a Spellcraft check DC 15 + <spell level>. That may not be 100% effective for higher level spells, but it is usually good enough.
We also adopted a system for casting straight from books at a hefty xp cost but no slot expenditure to make the idea of carrying a spare book around tempting.