I've downloaded from Amazon a few books using the Chrome Cloud Reader extension.
The only possible way to view those books is from inside the Chrome Reader extension , and let's be frank , it's pretty much a very annoying extension .
I want to view those books (I guess .mobi files) in my own viewer (Calibre) , but I can't find the books that chrome downloaded (I'm working offline so I downloaded the books , but they're invisible) .
Where is Chrome storing those offline books?
So this doesn't directly answer your question, but if you're trying to find the .mobi files for use with Calibre, I would just download the Kindle for PC application and find the downloaded files for importing into Calibre.
They're stored in...
Libraries/Documents/My Kindle Content~/Library/Containers/com.amazon.Kindle/Data/Library/Application Support/Kindle/My Kindle Content