Ian Wade G3NRW has made available as a free PDF his classic book NOSintro TCP/IP over Packet Radio
First published in 1992 NOSintro is an introduction to the KA9Q Network Operating System, the forerunner to JNOS, which is still in use today.
Although the book was written 25 years ago, much of the material is still relevant.
Download NOSintro - http://www.g3nrw.net/NOSintro/
Up-to-date discussions on JNOS - http://www.tapr.org/mailman/listinfo/nos-bbs