Most of the PPP RFCs are in the 1900s, but many of the important specifications are outside this range. You can find details about MLP in RFC 1990, LCPs in RFC 1661, Microsoft CHAP in RFC 2433, and CHAP itself in RFC 1994 at http://www.faqs.org/rfcs/.
Cisco hosts a number of design and implementation white papers on the topic of PPP at http://www.cisco.com/pcgi-bin/Support/browse/psp_view.pl?p=Internetworking:PPP.
The Internet Engineering Task Force (IETF) forms working groups to recommend specifications on technologies. The PPP extensions working group appears at http://www.ietf.org/html.charters/pppext-charter.html.
Most of the PPP RFCs are in the 1900s, but many of the important specifications are outside this range. You can find details about MLP in RFC 1990, LCPs in RFC 1661, Microsoft CHAP in RFC 2433, and CHAP itself in RFC 1994 at http://www.faqs.org/rfcs/.