I just got this book recently but I wish I had got it when I was first starting to learn linux as it would have saved me a lot of struggle. It answers a lot of the how to questions and will give you confidence in using the command shell.
This book is a great desk reference compendium of linux shell commands useful for all distributions.
This is a great MySQL book. Read through it and keep it handy as a reference.
This is a great MySQL book if you need to scale to a whole lot of users. Jeremy Zawodny is a super guru with real world experience squeezing the most out of MySQL for Yahoo!
This is the definitive book for PostgreSQL, the most advanced open source database.