This page provides links to all of the resources that I have found documenting the disk structures and design of the Reiser file system.
Unfortunately, the documents describing ReiserFS 3.x have been removed from Namesys's website. This document is an archived version of the whitepaper describing the philosophy, design, and disk structures of ReiserFS. Although the disk structures are not described as clearly as in Florian Buchholz's document, this is the most comprehensive description of ReiserFS available.
This is a chapter from a book written by Moshe Bar, which largely covers the same material as Han's whitepaper. In fact, most of the document is a direct quote of an older version of Han's paper--although the quote characters seem to be conveniently missing, and the information that is summarized is not entirely accurate. Nonetheless, the document provides a clear description of the concept of a 'unified namespace,' and has a few tidbits about the design decisions made in ReiserFS that are missing from the whitepaper above.
This is by far the most concise, complete, and accurate description I've found of the ReiserFS disk structures. Examples are given, the different key structures are explained, and the journal structures are defined.
Gerson is the author of rfstool. While this document is not as complete as Buchholz's, it explains the disk structures using excerpts from the ReiserFS source code. It also provides code snippets for enumerating all items within a leaf node, and for comparing keys.
If you are aware of any other documents describing the internals of the Reiser file system, please email the ReiserDriver mailing list so that they can be added here!