Most of the documentation is a snapshot of the old Namesys site (archive.org, 2007-09-29).
There was also a Reiser4 Wiki (archive.org, 2007-07-06) once on pub.namesys.com.
Reiser4 development appears to have stalled and ReiserFS is deprecated and scheduled to be removed from mainline Linux in 2025.
Mailinglists
From Reiser4 FS Wiki
You can:
See the VGER Majordomo site for more information.
Archives
There are several archives of reiserfs-devel on the net:
The archives of the former reiserfs-list can be found here: