My free online book on data compression.
http://mattmahoney.net/dc/dce.html

I may add some more stuff later. For now it covers most of the basic algorithms like LZ77, BWT, PPM, CM, Huffman, arithmetic coding, JPEG, MPEG, audio, etc, plus some theory. Most of the books on data compression are out of date, so someone needed to write this. Comments welcome.