The PPP RFC (the document that formally describes PPP): ftp.isi.edu/in-notes/rfc1661.txt. Have a look also at www.iro.umontreal.ca/~kropf/ift-6052/notes/ppp/ppp-rfc.html to see how the protocol evolved.
The URL with the latest version of pppd: ftp://cs.anu.edu.au/pub/software/ppp.
The PPP HOWTO: www.linuxdoc.org/HOWTO/PPP-HOWTO/index.html.
The IP_Masquerade HOWTO, if you want to create a home network that shares your PPP connection: www.linuxdoc.org/HOWTO/IP-Masquerade-HOWTO.html.
