"Xenology - An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization" is the almost complete, free online version of a comprehensive resource book for sci-fi writers and other interested people. It's from the 1970s, so the science isn't entirely accurate anymore (especially fields like biochemistry and exobiology were still in their infancy back then), but I think it's still very useful and interesting. It's written by a guy who now researches nanotechnology and who was involved with SETI and political advocacy for space exploration, so I'm fairly confident he researched this as well as he could back then. I haven't read all of it yet, but judging from the fields that aren't my speciality, it seems understandable enough for laypeople.
From the Summary:
"Topics include the history of the idea of extraterrestrial life; comparative planetology, stars, and galaxies; xenobiology (definition/origin of life, exotic biochemistries, and possible alien bioenergetics, biomechanics, sensations, reproduction, and intelligence); extraterrestrial civilizations (energy sources, biotechnology, interstellar travel, alien weapons, planetary and stellar engineering, xenosociology, and extraterrestrial governments and culture); interstellar communication techniques; and the sociology, legal issues, and appropriate interaction protocols pertaining to first contact."
(Edited for spelling and clarification.)