The protocol TACACS+ is documented in an internet draft. As of 1998-04-01, the current version can be found at ftp://ds.internic.net/internet-drafts/draft-grant-tacacs-02.txt. That draft contains some extensions (e.g. SENDAUTH) to earlier versions of the protocol which might not be implemented in all daemons, but should be backward compatible.
There are two mailing lists which may be of interest to users of TACACS+.
The first is a mailing list run by spot.Colorado.EDU which discusses many things pertaining to Cisco products. Requests to be added to or deleted from the list at spot.Colorado.EDU, along with other administrative issues concerning it can be sent to:
cisco-request@spot.Colorado.EDU
This is gated into the newsgroup comp.dcom.sys.cisco, but not vice versa. So your mails will show up as news, but you will not receive others' news articles as mail.
Posting should be directed to cisco@spot.colorado.edu, not to the request aliases.
There is also a list called TACPLUS-L, run by disaster.com, created for the purpose of information exchange between TACACS+ Users. It is intended as a supplement to the list at spot.Colorado.EDU, aiding TACACS+ users and prospective users in many issues including but not limited to technical support, bug reports and workarounds, configuration information, recommendations for future versions of TACACS+, and general talk about TACACS+ development, implementation, administration, etc. Refer to http://www.disaster.com/tacplus/ for further information, including how to subscribe.
Neither of these lists is in fact connected with Cisco Systems, Inc. or any of its subsidiaries. However, many knowledgeable people, including staff members of Cisco Systems, Inc. voluntarily read and respond on the lists. Standard etiquette rules apply.
Please do not cross-post both to spot.Colorado.EDU / comp.dcom.sys.cisco and the tacplus-l. Questions related to TACACS+ normally are most appropriate in tacplus-l, as the relevant audience is mostly found there also.