This list is in the best reading order I could find. It's not necessarily easiest to hardest, but based on subject matter.
Programming from the Ground Up by Jonathan Bartlett
Introduction to Algorithms by Thomas H. Cormen, Charles E. Leiserson, and Ronald L. Rivest
The Art of Computer Programming by Donald Knuth (3 volume set - volume 1 is the most important)
Programming Languages by Samuel N. Kamin
Modern Operating Systems by Andrew Tanenbaum
Linkers and Loaders by John Levine
Computer Organization and Design: The Hardware/Software Interface by David Patterson and John Hennessy