Hmm, that's another time I've seen that...
Quick note: Buying Wish spells is cheaper then buying those books.
Observe:
The price of a Wish spell: 26530 Gp (caster level * spell level * 10; from the services table in the PHB, assume a 17th level Wizard for maximum savings)
The price of 5 Wish spells (in rapid succession, presumably): 132650
The price of a +5 book: 137500
The difference: 4850 Gp
At Epic level, you probably don't care, but note that it goes up to a full 29100 Gp if you're planning on boosting all 6. And that's one more minor item in your arsenal... that just might make all the difference. Or not. Use as you will.