Picking a really helpful commentary on a Biblical book can be confusing. First, I don’t recommend you just pick a series and buy the whole thing. You will wind up with a very uneven collection where some volumes are great, and others are not helpful. This quick list with links will help you find the best secondary resources when studying God’s word. BestCommentaries.com – this is a hugely helpful website that is maintained by folks from Dallas Theological Seminary. The site rates commentaries for each book and even labels what sort of audience the book is aimed at. I recommend this for all of my students as a starting point in research. Bible.org – there is a part of this website where they have resources available for studying each individual book, or passage. I’ve found some of these to be quite helpful (such as the works from Eugene Merrill).