Book by
Piers Anthony.
Deals with our green-skinned hero, Flint of Outworld, attempting to thwart the threat of invasion of our galaxy by Galaxy Andromeda...his high Kirlian aura allows him to possess other people's bodies for months at a time with the proper equipment. So, he acts as an ambassador to other races, trying to get them to band together and stop the Andromedan invasion.
Much less hokey then it sounds--one of the greatest books of all-time. Later expanded into a series which followed various decendants of Flint on their journeys.