@linguistory @anj I would also recommend MarcEdit for transformations and also just for looking at files if you want to start to see patterns. You can also export a subset of fields into a tab-separated value, which I find extremely helpful.
There are also some built-in transforms.
If you want reference, I recc: https://www.loc.gov/marc/bibliographic/index.html
Looking at other stuff in the thread pymarc's labeled fields are definitely helpful for just getting some basics out of each, which MarcEdit doesn't support.