The following are some freely available computer sceince and Information Technology related reference text books.

1. Structure and Interpretation of Computer Programs
2. How to Design Programs - An interoduction to computing and programming
3.  Introdcution to Computer Science - Introduction to Computing and the Web
4.  Object Oriented System Development - written for professionals involved in the development of medium, large, and distributed systems and gives instructions on how to put key object-oriented concepts to work in software construction
5.  Building skills in object oriented design - for programmers who are new to OO programming
6.  Lecture Notes on Operating Systems - Lecture note on operating systems from computer Science Department University of Wisconsin - Madison.