While GH's site (and others that cross-post Nortel documentation) are very helpful, only the official Nortel site at
Nortel: Technical Support: Home will give you the most current versions of all documents. Trying to administer a CS 1000 5.5 system with a Meridian 1 25.40B document is not a path to success.
Contrary to misconception, the NTPs on
Nortel: Technical Support: Home are available to both end-users and channel partners. Registration is required to access the documentation.