Personally, I would love to have a good reference book for PDN on hand. It'd be handy to have and also I would like to learn more about how the program works in various chapters. I only know a bit about programming like in C++ but it'd be nice to get a full understanding of all the adjustments, blend modes and effects. Something that goes beyond a general description of what it does. For instance, recently I just got how levels and curves works and I've been using the program for almost a year now. It'd be nice to have a book that is a little more extensive and organized than the forums. I'd pay a good chunk of change for it.
So Ash...get that pen moving buddy! :-P
Also, it would be a fun project if we all worked together to contribute to a single published book, something for good reference use.