Originally Posted by NiLuJe
It's the SHA1 sum of the full filepath of the book. (ie. /mnt/us/documents/my_ebook.mobi).
AFAIR, it's been discussed before in the Kindle Dev & Calibre sections
On the other hand, Amazon books use something based on their ASIN (which I guess stands for Amazon Serial Identification Number).
As always, NiLuJe comes to my rescue.
I'd tried sha1 (since it's 40 characters) on file name and name without suffix, but never thought to try the full path.