When writing good Bible Studies, it can be tremendously helpful to look at a commentary on the passage you are studying. For example, it can be very helpful to read what Craig Blomberg said in his commentary on Matthew about Matthew 6:1-18. But which commentaries should you read? See below some links to some lists of recommended commentaries for each book of the Bible.
I also recommend:
Glynn, John. Commentary and Reference Survey: A Comprehensive Guide to Biblical and Theological Resources. 9th ed. Kregel Academic & Professional, 2003. $13.
I started asking for commentaries for Christmas presents my senior year at Taylor and haven't stopped since!