Racket and Scheme Language References
Racket home page
The Racket Guide
The Scheme Programming Language (Dybvig)
An Introduction to Scheme and its Implementation
An introductory programming book based on Scheme
Schemers.org
C Language References
The C Programming Language
by Kernighan and Ritchie. Alas, not free, but one of the best books ever written describing a programming language.
The C Book
C Tutorial
C Programming
C Programming
Programming in C
Valgrind documentation
Makefile tutorial
Git References
Pro Git
Git Immersion
Easy Version Control with Git