There are three former fanfic writers I know of who are published in book-length fiction, though I don't think their books are derived from fanfiction.
Debra Mullins writes historical romance fiction for Avon.
Leanne Shawler writes historicals for Zebra and as
Celia May Hart she writes erotic romance for Kensington.
And
Erin Klingler wrote what's referred to as a traditional or sweet romance for Wind River Publishing based in her home state of Idaho. I read Erin's book and there were a number of places where I could pick out an L&C reference.
