I was going to say what alaskamatt said... I usually have one complete through the first draft, let it sit and start another, then go back to the first for a second draft and work on the second.
While the first draft of the current "finished" book was going on, I had ideas for openings of the next book and the one after that. So I wrote them before they jumped out of my head. Just the openings.
Usually by the time the first one is really done, all cleaned up, the second one is about halfway through the first draft.
It's a constant process that keeps me sharp.
Of course, every now and then I need a day off. So I'll write a short story or something.