Andrew Coddington 
Aliens [PDF ebook] 

For as long as humans have been on Earth, they have wondered if other intelligent life exists in the universe. If so, what would these extraterrestrial beings look like? Today, there are true alien believers as well as skeptics. Some people have claimed aliens exist and have been responsible for creating great works of wonder, such as the pyramids and crop circles. Others are less sure. Regardless, human history has seen many stories of mysterious flashing lights, flying objects, and even alien abductions. This book discusses the history of alien lore and seeks to answer the question: Are humans alone, or are aliens among us?