I think the plan was for the next book to be The Olympian Affair. The YA series was (last I heard) going to be co-authored with his sister. The way he described the process sounded as though they would collaborate on an outline and he'd move on (meaning his sister would do the writing).
I'm glad to hear it. I love Jim, but I don't think he's good at writing Maggie--even in "Zoo Day" she doesn't sound like a traumatized child to me. She's not nearly enough of a pain in the butt for even a child raised in nice, typical surroundings, and kids who have been through rough times often have behavioral issues. I hope his sister can make Maggie a little less ideal.