A small detail that has been missed by most responses to the question is OP isn't asking how to learn how to write the best possible 2015 C code for 2015 C tasks, but asked for reverse engineering previously written code (perhaps written VERY long ago).
Depending on the age and quality of the code OP is reverse engineering, learning a very modern style of coding might somewhat oppose OPs goal.
I'd advise OP to rephrase the question to something like "I really want to learn how unixv6 works as a retro computing hobby project and I want to know what books the authors of unixv6 originally read to learn C". Or "I want to eventually be able to reverse engineer stereotypical 90s/00s microcontroller code so I want to know what textbooks an average EE used to learn C in that era"
Depending on the age and quality of the code OP is reverse engineering, learning a very modern style of coding might somewhat oppose OPs goal.
I'd advise OP to rephrase the question to something like "I really want to learn how unixv6 works as a retro computing hobby project and I want to know what books the authors of unixv6 originally read to learn C". Or "I want to eventually be able to reverse engineer stereotypical 90s/00s microcontroller code so I want to know what textbooks an average EE used to learn C in that era"