256-Color VGA Programming in C
VGA basics, lines and shapes, bitmaps, mouse, double buffering, page flipping, and unchained mode. All examples have code provided for Borland C, Turbo C, and DJGPP.
A to Z of C
The non-profit free book on C/DOS/Turbo C programming with 79 chapters. It's available online to view/download.
Pacific C
Freeware C compiler for DOS from HI-TECH software.