I observed in This thread (http://boards.straightdope.com/sdmb/showthread.php?t=712691 ) that Robert Bloch seems to have had a serious Jack the Ripper fixation. He wrote several stories about him, most notably his classic yours Truly, Jack the Ripper, but enough other ones for someone to assemble an anthology of Robert Bloch Jack the Ripper Stories ( http://subterraneanpress.com/store/product_detail/yours_truly_jack_the_ripper )
I’ve also noticed that Philip Jose Farmer had a severe “Tarzan” jones. It went beyond his whole “Wold Newton” nonsense - he had more Tarzan-related stuff than any other character in the universe:
Tarzan Alive! – the biography of Lord Greystoke
the Adventure of the Peerless Peer – Tarzan meets Sherlock Holmes (written at the peak of the Sherlock Holmes meets ____ craze sparked by Nicholas Meyer’s “Seven per Cent Solution”
A feast Unknown – essentially “Tarzan Meets Doc Savage” in a pornographic thriller. Names changed to protect the guilty. this sparked:
Lord of the Trees/The Mad Goblin – the G-rated sequel to his X-rated original, originally published as an Ace double.
** Mother was a Lovely Beast** – essays on Tarzan
Lord Tyger – millionaire tries to recreate Tarzan by having a child raised in the jungle by “apes”.
Any other examples of such writerly obsession with characters not their own?
I don’t count things like Robert Goldsborough extending the series of Rex Stout’s Nero Wolfe novels, or John Gardner (and others) writing new James Bond novels. Extending an existing series with new entries I can understand. But the two above cases are certainly NOT instances of a writer simply adding to a series. They’re variations, generally not part of a series at all, and use the character in different ways.