This page aims to serve as an index of useful resources for researching telecom history.
telecomarchive.com has an expansive index of historic Bell System documents, including Bell System Practices (BSP) and Bell Laboratories Review (BLR), Schematic Drawings (SD), and more.
TCI maintains a number of invaluable resources, most notably their library of scanned documentation. There are many BSPs and other similar documents available there, but as with any trove of old information, finding what you're looking for can be difficult. The archive includes a few period finding aids:
Phreaknet is a hobbyist telephone network project. The Phreaknet website includes a huge list of resources with many websites, articles, etc.
While Facebook has some considerable downsides, the Telecom Archaeology group there has a huge wealth of information.
Several long-running mailing lists, now generally on groups.io, have multi-year archives of Q&A discussion and folks sharing new resources. Searching these lists is often a great way to find information and anecdotes.
The Exchange Names Project was a Yahoo group collecting directory pages and other evidence of historic telephone exchange names. The group didn't survive Yahoo Groups turmoil, but I came into possession of the at least some of the files they had collected: browse them here.