Where are books downloaded from Amazon Kindle stored?
On my tablet (non-rooted) it exists at

storage\emulated\0\Android\data\com.amazon.kindle\files\

Mine has about 500 folders, which, I assume is one per book.
The names are some 10-character string of the form Bxxxxxxxxx, where x is alphanumeric. Inside each are a few files devoted to the book, probably the text in the file Bxxxxxxxxx_ebok.prc (it has the largest size), and a couple smaller files tracking reading progress, etc.

I have not been able to determine a translation between the coded folder and file names to the actual book, though I would, if I had tracked my download progress, have a rough idea based on calendar.