Took a while, but I didn't forget.

JtS likes "Power Screenwriting" (The 12 Stages of Story Development), by Michael Chase Walker the best.

His second favorite is "Creating Unforgettable Characters", by Linda Seger.

He has used "How to Write a Movie in 21 Days", by Viki King, but he says it's really only useful if you're stuck and you need a bit of a push.