My suggestion is that you have a strategy for you to master your job using SQL. A sample strategy would be:
0-Get a training environment, with the database, data and security
1-Learn principles of SQL
2-Get familiar with your product's data model
3-Jot down the top 50 queries your customer may require
4-Learn advanced SQL
5-Learn more about how to optimize your queries
6-Learn stored procedures language if you have to (such as PL/SQL, T-SQL,etc.)
Make sure that whatever book you use is for the same database you have. Don't pick a book that talks about ORACLE if you are using MySQL.
For point (1), I suggest you start very slowly so that you learn well and also so that you don't "hate" the subject.
Stay away from books with titles containing "complete guide", "bible", "coockbook", such books may be useful when you covered the basics.
Sample title that may suite you are:
(1)-SQL Visual Guide
(2)-SQL Queries for mere mortals
(3)-Sams Teach Yourself SQL in One Hour a Day (5th Edition)
(4)-The Practical SQL Handbook: Using SQL Variants (4th Edition)
Many other exist of course.
Read the book ratings and if you have a chance browse the book.
It would also help if you could afford (or if your company could afford) buying a SQL Query Helper tool that features Intellisense-like code completion.
A good book for one person is not always good for another person.
Hope this helps.