This is probably the strangest set-up for a trilogy I've ever read. The gap between the first book and the second means the first seems quite done once you have finished it. If I set down the series (and I do occasionally), I do it between Sabriel and Lirael, since it seems quite a natural pause. I do enjoy picking up the second and discovering that the characters from Sabriel have grown to become legends in their own time.
And spying on Sabriel and Touchstone with Lirael is awesome and makes me smile.