Extraterrestrial life IMO absolutely exists and so many ancient religious texts around the world blatantly talk about spaceships, 'gods,' technology, advanced weapons, etc. I'm in the camp that believes that human beings are the result of genetic engineering and not pure evolution. I also think that human civilization on this planet is far older than 6,000-7,000 years but unfortunately mainstream archaeology/history largely refuses to acknowledge newer findings that indicate that what we know is largely incomplete and/or just plain wrong. Anyone who is into learning about old megalithic structures would agree that it's highly damn unlikely that a bunch of humans thousands and thousands of years ago erected these sites/structures without some sort of help, especially quarrying and fashioning stones that were harder than tools available to them at the time that are cut so perfectly that you can't even slide a credit card between them... not to mention the fact that many of them are so heavy that even with modern machinery it would be nearly impossible to move some of them.
I have read some of these books as well.
The older I get the more I think of it as absolute bogus.
We have a clear timeline as is, in which everything fits. From simple life to advanced civilization, it all takes place in steps. There's no indication of a clear cut in history, a specific year or very short period in time in which civilization suddenly takes a hundred steps at once.. It's all very gradual.
Building shit with big boulders is also very primitive. It's far easier to do than building with small bricks, particularly because they had no cement and shit. The only way to build bigger things, than simple huts, was by using stones, preferably BIG stones.
Would aliens use these massive rocks to build things for them? I think not. They'd probably use steel and iron like we do nowadays, or some other alien technology. But it's really just big stones. Stones that have been collected at random: big and small, all oddly shaped.. surely us moderners and aliens alike would cut all these stones neatly into the same practical shape (completely square).
Considering that building with huge boulders is all they did.. they probably had better ways of handling them than we do today. We have grown so accustomed to machines that we wouldn't know how to take on such a massive task with our bare hands. Much like we also have 'un-learned' advanced hunting techniques.
And all that besides, there are only few places that have really nicely cut building blocks. Most of Europe's stone age monuments are nothing more than these boulders placed upright.. or in really really simple constructions, like this:
That's about the most advanced building that was made with big rocks in the northern parts of Europe. If aliens did that... it must have been the ones in the first grade of the special needs class.