Though not an official standard, the RADIUS specification is maintained by a working group of the IETF.

Guarding the Flank With RADIUS & TACACS+
An article from Network Computing.

Remote access using RADIUS and TACACS
PC Week article (August 1997) on the benefits and disadvantages of TACACS and RADIUS as open protocol standards.

Remote Network Access Security in an Open Systems Environment
White paper by Livingston Enterprises that describes the RADIUS protocol.

Web-Access Authentication Using RADIUS
Detailed article (June, 1996) from Web Techniques Magazine that describes how RADIUS can be used to add new types of Web authentication.