There is an ARRL book on the digital modes JT65 and JT9. These are among my favourite modes and ideally suited to QRP. I have used JT9 on bands up to 10m. I have used JT65 on bands up to 6m.
See http://www.arrl.org/shop/Work-the-World-with-JT65-and-JT9/ .
At the moment there is a special ARRL deal so you get this book free when you buy a new ARRL Handbook.