A Feast Unknown
1969
Not for the squeamish but a great, great book. Two of Farmer's all time favorite characters, Tarzan and Doc Savage (called Grandrith and Caliban here), do their best to kill each other. There is one problem though, they are being manipulated by a secret group called The Nine and every act of violence results in an erection and to kill results in an orgasm. Farmer does have an explanation for this and has a lot of fun with history in the process. This book has everything that Farmer does well, a must read.

Lord of the Trees/The Mad Goblin
1969
The sequel(s) to A Feast Unknown. Where as the previous book was rated X these would be rated R for violence (maybe PG-13). Tarzan and Doc Savage realize that they are not enemies and set out to get their revenge. The two stories run simultaneously, our two heroes separate at the beginning of the books and then reunite at the end, each story told from a different point of view. A very interesting idea and the stories themselves are a lot of fun.

The Monster on Hold
1983
Another possible title for this story was "Some Unspeakable Threshold". This is the first draft of a chapter from a Doc Caliban book that was never published (or finished as far as I know). This story would have picked up where Lord of the Trees/Mad Goblin left off. Now that four of the Nine are dead Doc discovers that they have a monster suspended between worlds, or univeres, that is breeding children to be sent after Doc and his friends. If the children fail the Nine may send the monster after Doc so he decides to make the first move and attack the monster.