NTP Documentation

4.2.8-series: CURRENT RELEASE: applicable to NTP version 4.2.8 through 4.2.8p18.
4.2.6-series: applicable to NTP version 4.2.6 through 4.2.6p5.
4.2.4-series: applicable to NTP version 4.2.4 through 4.2.4p8.
4.2.2-series: applicable to NTP version 4.2.2 through 4.2.2p4.
4.2.0
4.1.2
4.1.1
4.1.0
3-5.93e

Reference Library: Whitepapers, RFCs, and other technical documents written by Professor David L. Mills, inventor of NTP.
NTP Wiki: Community-contributed documentation.
NTP FAQ:Frequently Asked Questions about the NTP protocol and software distribution.
Public Time Server Lists: Browse or search for a public primary or secondary time server.
NTP mailing lists: Includes postings going back to June, 2003.