Isaac Asimov's Guide To The Bible. An agnostic atheist historian analyzes the history of every line of every book in the bible. Asimov is a good writer and he makes the bible a fun read.
I don't think I've seen these words used together. I've always understood the difference to be that an atheist is as sure in his beliefs as the religious zealot, each with the same amount of proof backing their opinions up. An agnostic realizing this lack of proof simply says "I don't know".
Agnostics believe that in some way, metaphysical intelligences are involved in the creation or fabric of the universe, but the very nature and depth of this involvement is in varying degrees indecipherable, unknown, and not-yet-known. Agnosticism doesn't require either the acceptance or rejection of theism.
Put together it technically means the person doesn't believe in a theistic god but holds agnostic views about broader metaphysical concepts.
More colloquially I've noticed a lot of people use the atheist agnostic / agnostic label interchangeably to indicate they haven't made their mind up one way or the other, but it's possible to be either atheist agnostic or theist agnostic.
I would take it a bit further and say it is impossible to ever know. Even if a powerful being were to appear and perform miracles, I would claim that the evidence is still not sufficient. As Arthur C. Clarke says: “Any sufficiently advanced technology is indistinguishable from magic.”