Novelists using popular fictional characters found in the public domain is one way to increase their readership and sales. I'm not a lawyer, but a clear-headed discussion of the copyright laws involved is given here: http://is.gd/Skb752.

You've probably seen (and read) the new books of such popular fictional characters. The list includes Frankenstein's monster, Dracula, Long John Silver, Robin Hood, Robinson Crusoe, Sherlock Holmes, and, well, you get the idea. The new books coming out seem to run in cycles, perhaps as the current reading tastes change.

For a time I was writing historical fiction, and TREASURE ISLAND had an appeal as authors set out to write the sequel or prequel. I'm not sure how many of books based on the same character that readers would feel compelled to buy. At any rate, I didn't find my writing style suited to historical fiction, so I never felt the inclination to tackle such a book.

Sherlock Holmes has garnered a lot of attention. I enjoyed reading his adventures when I was younger. I wrote one unpublished short story that was a Sherlock Holmes pastiche more as an experiment than anything else. Right now, I don't have any plans to write such a book though I admire other writers who have the talent to pull it off. Laurie R. King's Mary Russell, student and later partner of Sherlock Holmes, series is a superb example.