The .mbp is a file that contains any bookmarks or the last page read. The actual book file will be .azw (or possible .azw1, .azw2, .azw3).
The "B001CSM08E" part of the book you mention above id's it as 'The Forgotten Ways: Reactivating the Missional Church' which I found by searching for that ID on Amazon.com, the ID is known as an ASIN. ASIN's can be found on a book's product page as part of the product details so if you're looking for a specific book just search for it on Amazon and then look for the ASIN in your Kindle Content folder. If you have a lot of books you want to add it's likely faster to just drag them all into Calibre and then delete the ones you don't want from the Calibre library.