He had a very varied career. As I mentioned in the comments of the last post, he famously wrote Venus of the Half Shell under the name of Kilgore Trout, basing the novel on the Kurt Vonnegut's descriptions of the previously fictional novel. He wrote a lot of similar pastiches. I remember reading one novella based on the Raffles character that also involved Holmes and Watson. He also invariably included a character in his books with the initials P.J.F.
I really liked the first two Riverworld books, and eagerly awaited the final book. When it finally arrived several years overdue, it turned out that it was split into two books, neither of which were as good. I suspect part of the problem may have been that Farmer was unable to come up with a truly satisfying explanation for Riverworld's existence. What he did come up with was incredibly convoluted and disappointing, at least to me. (Not nearly as disappointing as the SciFi miniseries, though, which bore only a passing resemblance to the original.) I should probably reread at some point, at least the first two.