Over the years, I've read of number of Graham Hancock's books, and have enjoyed his style, respected his research, and appreciated that he was far more reserved than many writing in the "ancient civilizations in contact with extraterrestrials" genre.
One of the "interesting factoids" that has come up in many books in the arena is that the Sphinx, a much older monument than the pyramids, is oriented so that it would have been looking at sunrise in the "age of Leo", some 10-12,000 years ago. I had assumed that the Sphinx/Pyramids stuff was going to dove-tail in with a perhaps more measured look at the "anomalies" of the Cydonia region on Mars than is usually found in the works of Hoagland et al. While this is an element of the book, it takes up perhaps only a quarter of the text, with the rest of it being astro-physical conjecture of a most unsettling sort.
Much of the first half of the book looks at Mars and asks how it got to be the battered dead planet that we see today. Half of Mars appears to have had its crust blasted off, and there are craters on its surface that are far in excess of the blast that caused the great Cretaceous-Tertiary extinction event on Earth. In a manner like the Shoemaker-Levy comet impacts seen on Jupiter in 1994, Hancock proposes that a large comet had a near head-on collision with Mars, and split into multiple chunks of various sizes, the largest literally peeling the crust off of half the planet.
Prior to this impact, Mars was a warmer, wet world, with oceans, rivers, and possibly even a breathable atmosphere. Very much like Earth.
Most of the book is Hancock's research on comets and asteroids. And how vulnerable Earth is to these. I hate to go into details that are still disturbing to me, but the second half of this book is SO steeped with a sense of DOOM that it can't help to depress the reader. The "Taurid stream" (which produces meteor showers twice a year) appears to be the "debris trail" of a monster comet whose only visible remnant is the comet Encke, but which is filled with massive "dark objects" (some of these reflect less than 4% of the light that hits them) which we wouldn't be able to see until hours before collision!
Hancock obviously spent a great deal of time cross-referencing dates from astronomical programs with geological and historical records, and comes up with some grim predictions. Not only does he suggest that the last ice age was ended by a massive impact, but that over the past 1500 years we've been in a rare quiet period, and may be soon entering into a phase where the Earth will be hit with Tunguska-or-bigger objects as many as 100 times a year.
Again, one of the things I've always liked about Hancock's books is the intensive research that he puts into them, and in this case, it's very dark. Needless to say, he's STRONGLY advocating an international "near-Earth object" search and on-going watch, but it's depressing to see how little money is being spent on this currently.
One of the "wow" things in here is his suggestion that what happened to Mars did not happen in the vastly deep past ... in fact, he suggests that the comet/asteroid bombardment that killed Mars was the same one that ended the last ice age on Earth, as recently as 17,000 years ago. And that the Sphinx (and other monuments of anomalous vintage) are markers set to draw our attention to this event.
Again, The Mars Mystery is NOT a "fun read". It has its moments of fascination, and is well done, but is horribly depressing. If you want to be scared shitless about the future of the planet, well here's a book for you. If you want to stay blissfully unaware of the dire threat we live under (literally, we could be bashed by a planet-ending object any day and have only hours of warning), you might want to skip this. If you feel up to it, Amazon has the paperback at 32% off, and their new/used vendors have copies of the hardcover "like new" for as little as $2.50 (plus shipping), plus it does appear to still be in print so your local bookstore will likely be able to hook you up with all the doom you can handle. Seriously, it's that unsettling.