I have an idea for a SaaS product I want to create, however, this product will store extremely sensitive data that needs to be encrypted at rest. The trouble is not so much the encryption, but the problem of securely storing the keys so that in the event the server was somehow compromised, the keys couldn't just be recovered and used to decrypt the database.

Are there any decent books to guides regarding database encryption, and in particular secure key storage? This seems to be a less than straightforward topic and something that is difficult to get right. I'm seeing multiple ways to attack such a system, but unable to come up with one that is secure enough to store highly confidential information.