Machine Architecture and Organization

The C Programming Language, Brian Kernighan and Dennis Ritchie. The classic tutorial and reference, by the original creator of the language and an early colleague. Concise but clear. Even the second edition is now slightly out of date (it describes the ANSI 1989/ISO C90 standard), but the small differences won't affect 2021 much. Book home page
C: A Reference Manual, Samuel P. Harbison and Guy L. Steele Jr. Firmly a reference rather than a tutorial, this one goes for completeness. The most recent fifth edition covers C99. Book home page
C by Dissection, Al Kelley and Ira Pohl. Recommended by some previous years' instructors, this textbook is based on an approach of reading complete programs. Book home page

The GNU C Reference Manual
The GNU C Programming Tutorial
(SGI) C Language Reference Manual
C11 standard (A near-final draft; the official version is not available for free.)
C Tutorial by Brian Kernighan (Historical interest only.)

Steve Summit's Introduction to C Programming
comp.lang.c FAQ (Learn a great deal about the esoteric parts of C. A good resource if you are having weird problems.)
GNU libc manual (The GNU implementation of the C standard library.)
GCC manual (Be sure to look at the appropriate version.)
Optimization of Computer Programs in C (How to make your programs faster in the real world.)
What's new in C9x (C99, nicknamed C9x before it was ratified, is the intermediate C standard after the 1989/1990 ANSI/ISO C, but before the latest 2011 standard.)