The UNIX Programming Environment
I found this dusty old book from my old library and decided to read it. Written by the fathers of UNIX, Brian W. Kernighan and Rob Pike in 1984, most content is still relevant to this days and shows many of the design principles and motivations behind modern UNIX based systems.
Of course some content can be considered legacy today, as how to create a text parser using Yacc and Lex, or creating obscure filters with sed and awk.