A publisher says a former top Navy SEAL sniper and best-selling author who was fatally shot at the weekend had a second book in the works.

Sharyn Rosenblum says Chris Kyle was working on "American Gun: A History of the U.S. in Ten Firearms" with co-author William Doyle. Rosenblum is a spokeswoman for publisher William Morrow.

Kyle's book, "American Sniper," written with Scott McEwen and Jim DeFelice, was released last January. As of Tuesday morning, "American Sniper" was Amazon's No. 1 seller.

Rosenblum says no release date has been set for the new book.

Kyle left the Navy in 2009 after four tours of duty in Iraq, where he earned a reputation as one of the military's most lethal snipers.

Eddie Ray Routh has been charged in Kyle's killing Saturday.