AVR Assembly with Servo/Motors/Etc
Currently, and for good reason, people dont do this in assembly much anymore. Most stuff can easily be done in other programming languages, like C. Maybe you want to look at Arduino (arduino.cc)

To learn more about hardware programming the AVR, you may want to look into any of the excellent books available on the topic, although most literature features C as a language. Most of the C oriented books will give you good hints at the programming. You may then use that in assembler. 

Of course, you should also look at the papers at Atmel. They have an extensive library online. 

Imho, unless you've proven C is not sufficient for your needs, you sould try that, and see how far you get. 

Apart from that, Barrett "Atmel AVR Microcontroller Primer" has a decend description of the registers.