[chaley]

Any new calibre code should use the "new_api", so you should use the first method you described to get a db object.

from calibre.library import db; 
db=db('Path to calibre library folder').new_api

Without the .new_api you get an instance of calibre.db.legacy.LibraryDatabase. Occasionally legacy methods are easier to use, but in my experience not very often.

A "new_api" object is an instance of calibre.db.cache.Cache. Look there for "public" methods, properties, and attributes. For example db.prefs is a method (line 609):

    @read_api
    def pref(self, name, default=None):
        ' Return the value for the specified preference or the value specified as ``default`` if the preference is not set. '
        return self.backend.prefs.get(name, default)

Finally, and offered as a hint, in most cases you should specify a default when using .get(). In calibre, preferences can be and often are None. For example,

db.prefs.get('categories_using_hierarchy')

will return None or a list, meaning you must test it. If you instead use

db.prefs.get('categories_using_hierarchy', [])

then you will always get a list, simplifying subsequent processing.