Here are resources ranging from player made pages about Isengard, to mud clients as well as various "what is a mud" sites.
The Isengard Grimoire is the oldest player made site. It is now being regularly updated and features specific area maps as well as suggestions and tips for creating a new character and becoming a better player.
The Isen Worship Page is another player made site. It features suggestions for player stats as well as biographies and pictures of players.
If you'd like to make a suggestion about something you'd like to see on Isengard or have any sort of question visit the Isengard Message Board. The board is regularly checked by both players and admins.
The most popular mud client for Wintel machines, is zMUD. It features advanced scripting, macros, triggers and everything else a good mud client should have.
Macintosh players should try Rapscallion, it is by far the best mac client available and provides for easy scripting, macros, triggers, journeys, channels, etc.
MUDdom was completed by a group of students from the MSc Electronic Information Management programme in the University of Sheffield. Included within are several philosophies on what MUDs are, a comprehensive history of MUDs, as well as ramblings on where they may go in the future.
MUDdex - this collections of articles and papers on mudding compiled by Lauren P. Burka discusses MUDs and their possible impacts on society, as well as covers many general questions those new to MUDs often have.
The MUDding Resource Collection is another collection of FAQs and papers on muds and mudding. It also includes a short list of clients, MUDlists, FTP sites, and is very bandwidth friendly for those of you with slower connections.
The Mud Connector is searchable site with listings of over 600 MUDs. It also has mud reviews, a connection checker, as well as links to other MUD related resources.