nRF24L01 Product Specification v2.0 [PDF, 1.0 MB]
Nordic’s product specification for the nRF24L01 is quite clear and useful, far more-so than the documentation for previous radios.
AT90USB128 Hardware Manual [PDF, 4.5 MB]
The link points to the most recent version of Atmel’s AT90USB64/128 hardware manual, currently document number 7593J as of this report. I mostly used version 7593G from March 2008, but the revision history in version J doesn’t show any difference in the parts I used on the SPI module.
This page by Stefan Engekle presents code for getting the nRF24L01 working on an ATMEGA168. It was invaluable to my initial research into the radio, and gave me a clear understanding of how to interact with the registers.
This is by the same author as the above link. It gives code for using SPI on the AVR. My SPI code is a copy and paste job from this page, with a few minor modifications.
Brennen Bell provides five tutorials for the nRF24L01, four of them with code for PIC and ARM. His tutorial 0 was especially useful for me because of its commentary on the radio’s registers, which provided information that was not in the product specification.