Some text files of interest are:
ChangeLog.txt
This lists all the changes made for the patch.
GamePrefs.ini (Read-Only, located in addon\config)
Above each setting is a description of what the setting does.
If you want to edit the settings in GamePrefs.ini, you'll need to copy the settings to LocalPrefs.ini and edit them there.
If you want to have settings for when playing BZ2 and different settings for the Editor, copy them to LocalPrefs_bzone.ini and to LocalPrefs_bz2edit.ini and edit them there.
render.cfg (located in the root folder) which you can use to adjust some graphics settings.
render_editor.cfg (located in the root folder) which you can use to adjust some graphics settings used in the Editor.
ModLog (WIP, not yet published) Created by Commando
This file is worth its weight in gold to modders. It is a MS Help file which categorizes a compilation of all the new features and settings in all the patches.
When he publishes it, be sure to grab it.