[jscarbo]

Open Calibre on your computer, then connect your device by USB. Calibre should automatically recognize your device and a new On Device column should appear in left side of the Calibre GUI with all the books on the device shown with a green check mark next to them.

Select any or all of them by left clicking on the first one then continue clicking on successive titles while holding down the shift or control key (depending on whether you want to delete all of them or only selected titles). Then click the down arrow next to the Remove button in Calibre and select the last option, Remove From Device. It's fast and easy. I don't use Calibre Companion but don't think it will make a difference in the process.

If Calibre can't find them, the only other solution I have is to install the ES File Explorer app from Google Play and look for them that way. I never recommend using Windows Explorer to search and delete files from your device. It's too easy to make mistakes. If you have to manually delete book files, you may find that the Clean Master app will help delete any left over files and cached data. I've never used it for this purpose so don't know if it will work for this.