What to do next

How about Arduino Programming Notebook by Brian Evans (free to download)
http://www.lulu.com/product/download/arduino-programming-notebook/3524028

or

Practical Arduino - Cool Projects for Open Source Hardware

Not opened it yet, but its on my desk.

you may also like Programming And Customizing The AVR Microcontroller

Programming Microcontrollers in C

Have both, but not got the motivation to get passed the second chapters yet....but i will soon! ;D