Sunset of the Gods by Steve White
Jason Thanou is back (26 Aug 2013 Book Pick) and things are getting even weirder in his trips to the past. He finds out that there are worse things than finding that the anicent Greek Gods were aliens mucking with human development. At least they were slowing dying out. One a mission to observe, from a lot closer viewpoint than he planned, the battle of Marathon, things go pear shaped right quick when he runs into Pan!