GLE,
I wrote them all together, kind of like the Lord of the Rings movies were written together. I stopped to do some editing on book one, but I left all of the editing on book two until after I'd finished book three.
It takes a lot of work to get a rough draft through the editing process, and though Book Two was finished last year, we didn't start the editing until this summer (because I was busy writing book three.)
That means it will take a lot of time, still, to get the books out--even though the rough drafts are done. Book two, for instance, is currently going through its seventh draft. After that, it needs to be copyedited and proofread, a process that will take several months.
Even once that's done, Tor needs to find a spot for it in the line-up. That's why it's probably not going to be out until next April at the soonest. Book three, then, we're hoping to release in November of next year.
Think like the movies, again. Though they filmed all three together, the editing process--and the special effects work--was such that they had to wait a year between releasing them.