disk drive about full. Library folder has over 100 GB
Is this the root-level /Library, or the user ~/Library ?

The former contains things installed by applications: this could include large data like audio sample instruments for Garageband/Logic. 

The user Library contains settings and preferences, but it can also contain iPhone and iPad backups, which can take up a lot of space. You can manage these in iTunes' preferences.

You can use an app like OmniDiskSweeper which will show you the contents of your disk, sorted by size. You can drill down through each folder and see what's taking up space, and delete files if you wish. However, remember that it is very easy to delete files that may be critical and necessary to your Mac.