Ancient Astronaut Theory is a theory that humankind has been visited by an advanced extraterrestrial race or even several races thousands of years ago and still continues to be visited.
The followers of Ancient Astronaut Theory refer to ancient texts, ancient history and evidence they find at various archaeological sites as proof of those visitations.
They believe that the extraterrestrial races visited the Earth even before it was populated and that it is possible, that even the creation of the human race itself was a result of the extraterrestrial interference.
My Favorite Ancient Astronaut Theory Authors

Erich von Däniken
One can perhaps call him “the father” of the Ancient Astronaut Theory. A Swiss writer and a worldwide traveler, he dedicated his whole life to researching the possibility of paleo-contact.
Von Däniken is a co-founder of the Archaeology, Astronautics and SETI Research Association (AAS RA) and his books have been translated into 32 languages and together have sold more than 63 million copies.
Some of the most popular books by Erich von Däniken :

Zecharia Sitchin
Researcher and writer, who made popular the theory of Annunaki, extraterrestrial beings who arrived to Earth from another planet in the Solar System, Nibiru and created humans and later the Sumerian civilization. According to Sitchin, Nibiro has been previously unknown because it has an highly elongated elliptical orbit and comes close to the Sun only once in every 3600 years. In his theory, he referenced numerous Sumerian and Akkadian clay tablets that he had studied all his life.
Some of the most popular books by Zecharia Sitchin :

Giorgio A. Tsoukalos
The producer and the face behind the popular TV show “Ancient Aliens”. Besides “Ancient Aliens”, he has appeared on numerous other shows on Travel Channel, History Channel, Sci-Fi Channel, National Geographic Channel, and so on.
He is the publisher of http://www.legendarytimes.com which is dedicated to UFOs in Ancient Times, Alien Visitations in the remote past, Alien archaeology, references in mythology and ancient scriptures, evolution, genetics, philosophy, exobiology and space exploration.

David Hatcher Childress
An American Author and avid traveler, researching evidence of paleo-contact around the world. He is a recognized expert not only on ancient civilizations and technology, but also on free energy, anti-gravity and UFOs.
Due to his extensive traveling and exploring even the most remote corners of the world, he is considered a real-life Indiana Jones by his many fans.
Some of the most popular books by David Hatcher Childress :

Graham Hancock
A British writer and journalist. He is known for his research of ancient civilisations, Earth changes, stone monuments or megaliths, altered states of consciousness, ancient myths, and astronomical or astrological data from the past. His books have sold more than seven million copies worldwide and have been translated into thirty languages.
Some of the most popular books by Graham Hancock :

Brien Foerster
Brien Foerster was raised on the west coast of Canada, where he developed a fascination of the Native people, their art and oral traditions. This led to his becoming a professional sculptor, after graduating with an honors from the University Of Victoria at the age of 25. This included the creation of 15 full size totem poles, dugout canoes, masks, bowls, boxes and other Native style works. In 1995 he moved to Maui, Hawaii, and was hired as assistant project manager for the building of the 62 ft double hull sailing canoe. Over the course of the next decade he explored Polynesia, looking for the source of the Polynesian people.
Peru became his next major area of interest. The study of the Inca culture led to his first book, A Brief History Of The Incas. Nowadays he is an author of numerous books about Inca history and culture, as well as some excellent guidebooks about Machu Picchu, Saqsaywaman, Tiwanaku and other mysterious ancient sites of Peru and Bolivia . (I personally used them in my trip and highly recommend!)
Brien Foerster is currently the assistant director of the Juan Navarro Museum in Paracas, Peru, where you can find the largest collection of elongated skulls. He is heavily involved in the research and the DNA testing of those skulls. Additionally, he is the creator of HiddenIncaTours.com. Not only can you read tons of interesting articles on this website, you can even book an actual tour in Peru and Bolivia, led by expert researchers and authors!
Some of the most popular books by Brien Foerster :
to be continued…