(83 karma - 2012) https://news.ycombinator.com/item?id=4624513
(40 karma - 2018) https://news.ycombinator.com/item?id=17803599
1) Internet System Handbook by Marshall Rose and Daniel Lynch - Overview of The Internet from the 1990s. Very valuable to get the big picture.
2) An Engineering Approach to Computer Networking: ATM Networks, the Internet, and the Telephone Network by S. Keshav - Excellent textbook which covers three different types of Networks.
3) The TCP/IP Guide by Charles Kozierok. - Reference: http://www.tcpipguide.com/ Start at "Networking Fundamentals" : http://www.tcpipguide.com/free/t_NetworkingFundamentals.htm
4) Mathematical Foundations of Computer Networking by S. Keshav - https://news.ycombinator.com/item?id=30964707
That book has done wonders for me.