This is basically a bump of a post from 10 years ago that came in handy again today. It’s worth remembering:
The TCP/IP Guide is a free (donations welcome) online text covering TCP/IP in depth. A thorough, easy-to-read reference to the TCP/IP protocol suite and miscellaneous internetworking topics. For those who’ve always wanted (or sometimes need) to know the details of TCP adaptive retransmission, or how BGP actually works.