Your modem manual (if it exists). Some modems come without manuals.
Serial Suite by Vern Hoxie is a collection of blurbs about the care and feeding of the Linux serial port plus some simple programs.
The Linux serial mailing list. To subscribe, send email to email@example.com, with ``subscribe linux-serial'' in the message body. If you send ``help'' in the message body, you get a help message. The server also serves many other Linux lists. Send the ``lists'' command for a list of mailing lists.
Rob Clark had a huge database of modems but it doesn't seem to have been maintained since 2003. The website URLs have changed many times. Right now (mid 2006), it seems that only these two mirrors work: