Free online "Intro to Computer Networks" book.
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I just found http://intronetworks.cs.luc.edu/
Welcome to the website for An Introduction to Computer Networks, a free and open general-purpose computer-networking textbook, complete with diagrams and exercises. It covers the LAN, internetworking and transport layers, focusing primarily on TCP/IP. Particular attention is paid to congestion; other special topics include queuing, real-time traffic, network management, security and the ns simulator.
I have been reading plenty of networking stuff lately, and this is one of the best books I have (partially) read. It does good job of explaining things without getting mired in the details. Also, the book is being actively updated, so the information should be rather current.
The author also is a supporter of open source and Creative Commons. :)