As with The League of Extraordinary Gentlemen, this unifying theory makes for a nice vehicle for literary analysis, criticism and subversion. Farmer's Doc Savage, His Apocalyptic Life is an authoritative account of Lester Dent's Doc Savage series of pulp novels, and the peripherally connected The Other Log of Phileas Fogg a neat reinvention of Around the World in Eighty Days, taking the inconsistencies and pecularities of Verne's novel and weaving them into a backstory about a conflict between two alien races. Tarzan, too, is a prominent figure, both in his conventional identity as Lord Greystoke in the biographical Tarzan Alive (see Tarzan Alive at Ed Stephan's Tarzan of the Internet) and as "Ras Tyger" in Lord Tyger, an unbowdlerized reimagining of Tarzan:
My Mother is an Ape
My Father is God
I come from the Land of Ghosts
So sings Ras Tyger, Philip José Farmer's superb incarnation of a modern-day Jungle Lord. He is fluent in four languages. He devours grubs, insects, and palpitating flesh. He communes with wild beasts and proffers them his love. Men he butchers. He is feared as a ghost, yet the village women welcome him at night. savage, heroic and beautiful, he is the master of his world - until the day when the incredible truth of his existence begins to unfold...
- cover blurb to the 1974 Panther Books edition
It's like a jungle Truman Show - Ras Tyger comes to the realisation that his whole environment is a construct, the scheme of a millionaire who wants to create a duplicate of the fictional Tarzan.
Lord Tyger also grades into another aspect of Farmer's work, its frank exploration of sexual themes as one of the "New Wave" SF authors. His first published story in 1952, The Lovers, set the scene with its story of a puritanically-conditioned linguist's relationship with an uninhibited humanoid woman who turns out to be an insectoid mimic: a miscegenation that "nauseated" John W Campbell but didn't stop the story winning a 1953 Hugo Award when published elsewhere. This vein continued in various guises throughout his career: Flesh, in which a starship captain takes on the role of a Golden Bough style priapic god; Strange Relations, a lower-key exploration of family-like aspects of alien-human interactions (see the Addendum); the erotic horror of A Feast Unknown (which brings Wold Newton characters into a pastiche making explicit the homoerotic subtexts of the action hero genre); and the borderline-pornographic Image of the Beast and Blown, which again involve classic fictional characters.
As with many authors, some of Farmer's work tended toward the potboiling. I found his "World of Tiers" parallel-world series unexceptional, and the superb and also Hugo-winning To Your Scattered Bodies Go (in which Sir Richard Burton, Alice Liddell and other historical figures interact in a seeming afterlife among all humanity on the shores of an endlessly stretching river) also stretched seemingly endlessly to the far too many sequels of the Riverworld series. When he cut to the chase, however, Farmer was invariably readable and inventive. He was also a brilliant and adaptable prose stylist, an aspect that shone in such conceits as The Jungle Rot Kid on the Nod, a short story about Tarzan as if it had been written by William Burroughs rather than Edgar Rice Burroughs.
For more background, see the official Philip José Farmer Home Page (The Brobdingnagian collection of all things Farmerian) and the Philip José Farmer International Bibliography.
Addendum (topic upgraded from comments): Felix Grant just reminded me of our conversation a while back of Farmer's story Mother, from Strange Relations. In The Lovers, Farmer had introduced one example of a strange reproductive biology, in which mimicry is achieved by the mother literally "photographing" the father at the moment of conception. Strange Relations is a story collection not so much about sex as about human encounters with other unusual reproductive biologies. Mother, for instance, tells of a mother-dominated man who survives a starship crash, but finds himself literally returning to the womb when he is trapped inside the comfortable interior of the sessile female breeding stage (the "Mother") of an alien life-form that relies on non-sessile creatures ("mobiles") for food and pollination-style fertilisation.