I would recommend Reading List plugin. While I don't know anything about emailing books, this plugin has a tab for Device, surely it should be good for something. Also it seems like you can populate the list from the device.
What I know for sure: You can create a list named 'in Cloud', and make it so that every time you add a book to this list (simple process, 2 clicks), it enters your tag into your column. Or pops up with a message saying this book already is in the list.
Not good practice: Deleting the tag manually - book still stays on the list. Entering the tag manually (like before plugin was installed) - book needs to be put on list anyway.
Then when you have tags in place, you can use it and color the title purple!
(for quick reference)