It is expensive to build a library!
Oftentimes, young students in Bible & Theology are overwhelmed with the number of books that exist in this subject area, and rightly so.
New ministers are often tempted to "buy" lots of books - and amass a library of good resources. While it would be great to build a great library - the costs prohibit it as a reasonable reality for most pastors. And, what is more, most pastors amass libraries that are larger than their competence, beyond the scope of their actual use, more than what they have actually read! [Yes, pastors buy books to put on the shelf, which they have never read, which they may never read!]
I would rather pastors invest in only the books they (1) actually read and/or (2) will actually use.
Case in point - years ago, I owned numerous commentaries on the New Testament epistles. I spent hard earned money on those commentaries. In the course of time (my work as a Scholar and my engagement on University campuses), I realized I rarely read from or used the commentaries I owned on the New Testament epistles. And, on the infrequent occasion that I needed good commentary material, I had a University library easily at my disposal. My hard earned money on those commentaries was, to an extent, wasted.
As a result, I have learned to discourage new ministers from rapidly buying many books or commentaries - and instead, encourage them to buy books and commentaries they will use. I typically advise something like this:
"Instead of buying an entire Commentary Series on the New Testament, consider buying five to seven, maybe eight of the best commentaries on, let's say, Luke's Gospel. Instead of owning 26 commentaries that you're not using on the other Gospels, Epistles, Acts and Revelation - build your library book-by-book instead of series-by-series, with resources you actively & deliberately use. I would rather you own five to eight well-read and deliberately used commentaries on Luke, than a shelf full of commentaries you've never read! And, this is better stewardship of your finances - and shelf space! [And, owning books you read is easier to justify when you move and have to box them all up - compared with owning books you might-or-might-not one day read!]"
Of course, you have to buy textbooks for courses you take. Hopefully you will have been encouraged to read and own great books. [Professors that I know genuinely and intentionally decide on the best texts to use in the classroom.]
As you think about building your library though - let me encourage you to be a wise steward - in your purchases - both at the point of the cash involved - and at the point of real reading and extended future use.
Also, be mindful that theological commentaries have take up positions on matters specific to religious discourse. Said another way, you should know that Mary-Knoll-Orbis, a great publisher, is a Roman Catholic publisher and therefore espouses views and uses ideas more akin to Roman Catholic concern. Abingdon Press is a Wesleyan publisher. Inter-Varsity Press is generally evangelical, but not tied to any one denomination. These things matter, and there are lots of different publishers. In time, you'll learn how to discern these issues.
With this in mind, let me also link you to this well-reviewed, collegial and refereed perspective of the "Best Commentaries" to use. As you will note in life and in scholarship, the specific publishers and the specific authors have presuppositional theological commitments, but this website, with the vast array of contributors and perspectives, seems to give a truly excellent, balanced review on "the" Best Commentaries.
Blessings to you and your work in the libraries that shape our discernment of the Bible & Theology!