For information on MySQL, I usually refer to the documentation on the MySQL web site, http://dev.mysql.com/doc.

For information on R, I refer to the R web site, http://cran.r-project.org/manuals.html. PDF copies of the manuals listed on the web site are usually installed with R, so check the Help menu in R to see if you already have them on your computer.

For information on Perl, I usually refer to Programming Perl (O’Reilly).