Wow! So much designs. Guess Mr. Rick Warren and his crew are gonna have more work choosing then all us designing :).

Anyway, 99dsgns said it clearly: NO STOCK, if for nothing else - because of licensing - You Can Not Give Your License to the customer. And also, this book will probably be printed in few tens-of-thousands copies at least, so, normal, royalty free photo from micro-stock sites (10$) is NO GOOD. You must purchase EXTENDED license (70 - 150$), or, not You, actually, Mr. Rick Warren must buy that license if he wants to use it on his book cover.
So, - no stock!

This is my understanding of things in this matter, just trying to help ... :)