If there is a complete set of commentaries on the Bible, should the classification 220.7 be used for each book? If commentaries are individual books on an old testament book or two, should they be classified as 221.7?
Alice, I don't know what happened. I responded to your question addressed to me and also to this message and I find neither of them. So, I'll respond again!
You mentioned that you were classifying your pastor's library. I, too, classify my pastor's library and the following is how I classify his as well as in our church library.
You will find more detailed information in our loose-leaf notebook entitled The Church Library Ministry Information Service.
This is what is recommended. If there are 12 or fewer in the set, they should be given the commentary number--220.7 if on the entire Bible, 221.7 if only on the Old Testament, and 225.7 if in the New Testament.
If there are more than 12 books, it is best to classify according to the individual book using the number which is assigned to every book of the Bible. If more than one book is covered, then use the number which covers all the books--an example for the O. T. would be to use 222.3 if the commentary covered both Judges and Ruth. A N. T. example would be to use 226.1 for a commentary covering Matthew, Mark, Luke and John. You can see the pattern from the book A Classification System for Church Libraries.
One advantage of using the above classification is if someone is studying Psalms, they can find all together the books on Psalms and are more likely to check out the commentary than if it was with a set of commentaries.
Hope this clarifies your question.