A fast read that was an improvement on the tepid book 3 that almost made me gave up on the series; as usual in his more recent works, H. Turtledove is focusing at the grunt level rather than the movers and shakers, so you get a "low level" view of the action, characters sometimes die but new ones replace them, while his alternate timeline chugs on.

You may or may not buy the author's premise, but his world building is logical if you accept them and the books read like historical novels but for the changes in the timeline.