Irving, this is a great place to ask that question. To give you more useful suggestions, it would help to know what your current background is in some of the areas covered by complexity, but I’ll give some broad pointers and be more specific if you need.

I’ll mention some good general references/textbooks, some of which I’m sure were mentioned in Dr. Mitchell’s notes section in the back of her book, which is also a good source of some more literature of a technical nature.

Information theory: Cover & Thomas‘s text is iconic as is Claude Shannon’s original (short) paper. You might also try some of the basic and inexpensive texts by R. Ash, F. Reza, or S. Kullback.
Thermodynamics/entropy: H. Callen has one of the most standard and broadly used texts while I’d also highly recommend any of the entropy related books by Arieh Ben-Naim or the classics by E.T. Jaynes or L. Brillouin.
Microbiology, evolution, genetics: Alberts, et al. is a good standard start in addition to Darwin, of course.
Genetic algorithms: Dr. Mitchell has a good introductory text here
Cellular automata: John von Neumann has the original text in Theory of Self-reproducing Automata and S. Wolfram‘s text was also mentioned in Mitchell’s book as well.

For more specific journal articles and research papers (these can get very technical) you should try some of the foundational works in any of these areas. I personally maintain a fairly large collection of journal articles at the intersection of information theory and molecular biology including many of those mentioned in Dr. Mitchell’s “bibliography” on Mendeley.

Depending on how you came to my blog, you may have missed many of the other references and materials available from the front page.

Let me know if I can be of further help with more direct specifics.