If you buy a fourth book for
studying the Bible, buy a whole-Bible Commentary

Buy at ChristianBook.com

Commentaries are the fourth category of Bible study books.

Within the commentary category are the following subcategories:

1. Whole Bible Commentaries

2. Individuals Book Commentaries

3. Topical Commentaries

Overviews, themes, explanations, history, and usually a verse-by-verse analysis if based on the whole Bible or an individual book. Topical commentaries will collect and analyze all important verses on a topic.

The authors of commentaries did not write the Bible.  Their writings are not inspired like the Bible.  Poor commentaries block your understanding of the Bible by false teaching, something apostle Paul  warned us about.  

Knowing the good publishers is the first step in getting good commentaries.

Believer's Bible Commentary
By: William MacDonald
Editor: Art Farstad
Thomas Nelson, 1995 
ISBN: 0840719728
About $30.  
A single volume providing a basic commentary based on the New King James Version.  

King James Bible Commentary
By: E.G. Dobson, C.L. Feinberg, E. Hindson & Others
Thomas Nelson, 1999
ISBN: 0785246010 
About $20.  
A single volume for students, teachers, and pastors. Appears to provide greater depth than the Believer's Commentary.   

The NIV Bible Commentary, 
2 Volumes
By: Kenneth Barker John Kohlenberger
Zondervan Corp., 1994
ISBN: 0310578582 
About $50. 
This is the premier two-volume set in my opinion.

Baker Commentary on the Bible
Editor: Walter Elwell
Baker / Revell, 2000 
ISBN: 0801022541 
About $15
Top notch scholarship, helpful, and maybe the best value.