In the next couple of weeks, I will be giving a talk for the local Secular Student Alliance concerning how we go about trying to find extraterrestrials. A big part of that will be looking into the claims made on the History Channel about aliens in the past which have been rather under par for good history lately (and less lately). There are numerous good books and websites on the subject that undercuts the claims of ancient astronauts, from the Skeptic's dictionary to an older book by Ronald Story, The Space Gods Revealed.
Jason Colavito's The Cult of Alien Gods. It wonderfully goes through the literary sources of Erich von Daniken and his successors, but its major thesis is about how the ideas originate in the fictional works of American author H.P. Lovecraft. Colavito basically proves that Lovecraft is the source behind the works (mainly French) that would try to prove there were aliens in the distant past, and those were the sources utilized by von Daniken later on.
The book isn't a debunking book, though it does take the time occasionally the eviscerate the claims of von Daniken, Graham Hancock, and others. Another joy is that it also gives the historical background to Lovecraft's work (from the poetry of Poe to the Theosophical Society) as well as the cultural matrix that made the ancient astronaut ideas take off, especially in America.
You will learn not just about a pseudo-historical idea, but also about real history and literature. It does great work all around to make its thesis work and teach you many other things.
The only gripe I have is that it tends to blame a lot of this on postmodernism, which I find to be a weak thesis. There were crazy ideas like this before Derrida (Ignatius Donnelley was big in the 19th century with his Atlantis ideas), and most people are not reading 20th century philosophers or literary critics. It's more about what people want to believe (which Colavito notes), and the philosophical rationalizations are an afterthought for most, if they ever even consider it.
Try to get a copy of this book, and you will be able to see what is going on with the ancient astronauts as well as learn to appreciate the Cthulhu Mythos all the more. Well-researched and with end notes and sources (unlike some people...).