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.

AVRLib/nRF24L01 – TinkerWiki

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.

AVRLib/SPI – TinkerWiki

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.

DIY Embedded

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.