These are a few links that point to resources regarding MARC files and editing XML more generally. Do feel free to email me if you have any questions!
- Wonderfully dated MARC overview through Libraries Linking Idaho
- MARC reference guide
- OpenRefine Tutorials & Docs
- Owen Stephens' posts on converting between MarcEdit and OpenRefine included for completeness, no expectations that you be able to follow this tutorial unless interested
- OpenRefine
- MarcEdit
- Online XML Validator
- For Macs: TextWrangler
- For PCs: Notepad++