Only one novelization set in the Firefly Universe exist. That is the novelization of the film Serenity, written by Keith R. A. DeCandido? and published by Pocket Books in 2005. At the time that book was contracted, Pocket Books also contracted to publish two original novels, and solicited proposals from various authors. However, none of these proposals were approved by Joss Whedon, and after a year had elapsed, the contract was canceled.
Beside the novelization of Serenity two collections/compilations of essays about Firefly and Serenity exist.
Finding Serenity: Anti-heroes, Lost Shepherds and Space Hookers in Joss Whedon's Firefly (2004, edited by Jane Espenson? and Glenn Yeffeth?)
Serenity Found: More Unauthorized Essays on Joss Whedon's Firefly Universe (2007, edited by Jane Espenson)