I would start with Bash or just Shell Programming in general. I worked as a SA for many years on various UNIX platforms and was able to do 99% of what I wanted with shell scripts. For shell programming books I like UNIX Shell Programming and Mastering UNIX Shell Scripting. I would also suggest getting a Safari account to save a little money on books. Nothing worse than buying a $50 book then not needing it a year later and not being able to sell it. For Perl or anything UNIX O'Reilly will be your best start.