Sabriel is still my favourite. I don't know, she grabbed my heart more than Lirael did.

Only one story in Across the Wall is set in that world, and I don't think he intends to write more of that world, but he's been rather prolific about publishing nonetheless. The Keys to the Kingdom series come with a whole new set of phenomenal ideas, and while it's directed at perhaps a slightly younger audience, I had no shame in reading them religiously. :D