Biblical commentaries are a tremendous tool for understanding the Old and New Testaments. The problem is that there is not just one kind of Bible commentary. There are commentaries for preachers, devotional commentaries and scholarly commentaries. Some require some knowledge of Hebrew or Greek and some do not.
But you don’t want to wait until have purchased a commentary before discovering what kind of commentary it is.
Thankfully, you do not have to. One of the best resources on the web is a website called Best Commentaries. You can look up any book of the Bible and see a list of the best commentaries. Not only does this site rank the commentaries, it provides a description of the type of commentary it is.
If you are going to be doing any study on a book of the Bible, I recommend you visit Best Commentaries.