Good books regarding SQL and PL/SQL are written by Tom/Thomas Kyte and Steven Feuerstein.
Explore these links:
http://asktom.oracle.com/pls/asktom/f?p=100:1:0::NO:RP:: (book is on the right)
I have read the following ones:
- SQL: Oracle SQL Fundamentals I. Exam Guide. Published by Oracle Press. -> I like it becouse it gives you a great overview and you can find examples and exercises insde.
- PL/SQL: PL/SQL Programming. Author: Michael McLaughlin. Published by Oracle Press -> Structured as the previous one with examples and exercises at the end of each chapter.
- Shell Script: Linux Command line and Shell Scripting. Author: Richar Blumg. Publishcied by Wiley Publishing -> This book is organized in two parts: Linux commands and Shell Script and explains everything clearly. Ideal if you don't have a previous background. I find it a good book :-)
As Hoek sayas, Tom Kyte is the reference for everything related to SQL. It would be interesting that you check his books.