I've read the first two books of his "Axis of Time" series and enjoyed them. Also an alternate history story, basically the US Navy of 2020 (or so?) uses an experimental weapon which takes part of the fleet (literally, as some ships are bisected by the spherical nature of the of the effect) back in time to WWII. Ships are scattered all over, some landing in 1942's US Navy, some in Japan, some in other places of the world.
I haven't gotten around to book #3 yet, but the first two were very interesting, seeing the technologies clash, alliances form, and seeing the cultures of two eras butt heads against each other (e.g. how sailors of 1942 would find it unthinkable to receive orders from a black woman as a superior officer).
I'd never read any alternate history books before, but Birmingham has gotten me interested--very intelligently written and plausible. I'll be sure to check Without Warning out at some pont.