Books on optimizing and refactoring SQL stored procedures
The best two books here are both from Itzik Ben Gan - Inside SQL Server T-SQL Querying and Inside SQL Server T-SQL Programming.

Read the Querying one first. Covers all the important information on how queries are processed, thinking in terms of sets, etc. Covers all aspects of querying. The second book goes into programming constructs when working with T-SQL. Amazing books.