A commentary is a "collection of chapter-by-chapter or verse-by-verse explanations of either a portion or the whole of Scripture. Commentaries may be written by individual scholars, who are commonly assigned, based on their interests and expertise, to write on a partiular book." - David R. Stewart, The Literature of Theology
Commentaries may be published as a single volume or as a series.
Consider when selecting a commentary:
-Is it recognized by most respected scholars?
-At whom is this commentary aimed (preachers, scholars, etc)?
-Does the commentary interact with the original language?
-How old is the commentary?
Further assistance: Best Commentaries
While some commentaries are available in print only, there is some online access. We in the library can also scan/digitize a section in a any of our print books or reference works if you are a distance student. Use Primo to find the book you want and "Request PDF" or "Digitization."
Search for the term/passage you are seeking in Primo and add the word Commentary, like Luke commentary, or just the reference, like John 3 or John 3:16.
JSTOR eBooks database offers a number of eBook commentaries. Search either for the title of a commentary, or “Bible Commentary” – in quotes - and limit to the subject of religion. Try also “Commentary on” with the limits of Book or Book chapter and subject Religion.
Try Bible Gateway commentaries https://www.biblegateway.com/resources/commentaries/
And Bible Study tools https://www.biblestudytools.com/commentaries/ platform.
These two websites also provide parallel translations (to compare) and Greek and Hebrew word analysis for verses. Just search your verse, like John 3: 16 Greek, John 3:16 parallel, Genesis 1:1 Hebrew.
You will want to make sure you are using a quality source. Check out the Best Commentaries site.