If you're looking to go from the C angle, Gtk is the toolkit you'll be using.

http://wiki.maemo.org/Documentation/...eveloper_Guide

The majority of libraries available on the N900 are similar, if not identical, to their counterparts on most standard Linux desktops. It's less a question of what libraries there are and more "what do you want to make." From there you can probably identify which libraries you need and what is available on the N900.