NTP Project and Reference Library - Includes informational slides, project background, a bibliography, and an executive summary, in addition to links to more information.
NTP FAQ - Volunteer maintained. Includes information on both the software distribution and on the protocol.
Information on time and frequency services - Links to NTP servers and reference clocks, NTP software and documentation, time and frequency standard stations operated by U.S. and other countries, and time and frequency standard station transmission data are available here.