I've started with BookSwap (one of the stickied threads) and a bit of Sudoku, then switched to reading decompiled script files / experimenting.
You can decompile xsb and some .so files using this:
http://projects.mobileread.com/reade...k/DeFsk_02.zip
kbook.so and FskCache.so are probably the most interesting. You can extract them from firmware image yourself or just download them from PRS+ site:
http://code.google.com/p/prs-plus/so...ok/application