There are too many links to be listed here. The PDF version of the book contains most of those that I have used in its preparation. However, there are a few general links that are of such general usefulness that warrant special attention.
First, out of courtesy to the host, the Google Scholar - a search engine finetuned to scientific research
Secondly, to reflect the respect for the institution of preprints, which allow nonaffiliated researchers, especially those without access to specializes libraries, to read the current scientific literature.
The Arxiv.org gives an open access to e-prints in Physics, Mathematics, Computer Science and Quantitative Biology, often before they do get published.
Another option is to go to http://myweb.yahoo.com/ and search for countryofblindfolded which should point you to a lot of tags collected during the searches for the book material.
The next net resource of incalculable value is the Wikipedia, the free encyclopedia that anyone can edit.
For those interested in the full bibliography - many sources that have not been included in the book -
here is the bibtex file with references. I use JABREF reference manager http://jabref.sourceforge.net/