The JXTA platform is an open network computing platform designed for peer-to-peer (P2P) computing. JXTA is agnostic to programming languages. Therefore, one can start learning about JXTA from any of its implementations. JXSE is the Java implementation, with new extensions provided by JP2P.
JXSE offers programmer's guides and tutorial code. This is a good starting point for those who want to use JXTA in their applications.
Each recent release of JXSE has contained a Programmer's Guide.
2.7 Programmer's Guide [PDF]
2.6 Programmer's Guide [PDF]
2.5 Programmer's Guide [PDF]
If you are interested in the protocols themselves, consult the latest version of the Specification [PDF].
JXTA by Brendon Wilson, originally published by New Riders. Available from the author's site as PDF. Covers Java 1.3, and build 47b of JXTA. (2002)
JXTA in a Nutshell, by Scott Oaks, Bernard Traversat, Li Gong. Published by O'Reilly. (2002)
Mastering JXTA: Building Java Peer-to-Peer Applications, by Joseph D. Gradecki. Published by Wiley. (2002)
JXTA: Java P2P Programming [Amazon link] by Daniel Brookshier, Darren Govoni, Navaneeth Krishnan and Juan Carlos Soto. Published by SAMS. (2002)
Java P2P Unleashed: With JXTA, Web Services, XML, Jini, JavaSpaces and J2EE [Amazon link], by Robert Flenner, Michael Abbott, Toufic Boubez, Frank Cohen, Navaneeth Krishnan, Alan Moffet, Rajam Ramamurti, Bilal Siddiqui, Frank Sommers. Published by SAMS. (2002)
Early Adopter JXTA: Peer to peer computing with Java [Amazon link] by Sing Li. Published by Wrox. (2001)
Franck Besnard wrote Data Replication in a Multi-Cloud Environment using Hadoop and Peer-to-Peer technologies. (2013)