Apple - OS X Kindle App: Where are the books located?

These files are now (October 2012) stored in:

~/Library/Containers/com.amazon.Kindle/Data/Library/Application Support/Kindle/My Kindle Content/

If they move again you can find them by running the command:

find  . -name *.azw

The Kindle app stores AZW files, but only downloads them for Amazon-purchased content that you have opened in the Kindle app on that computer.

The AZW files are saved in ~/Documents/My Kindle Content.
Their names are random-looking (like B0043M4ZH0_EBOK), so it takes a little work to figure out which file corresponds to which book. Here's how you do that:

Open the book of interest on Amazon.com (make sure you're looking at the Kindle edition). It's URL will be something like this: http://www.amazon.com/Ultimate-Hitchhikers-Guide-Galaxy-ebook/dp/B0043M4ZH0/ref=tmm_kin_title_0?ie=UTF8&m=AG56TWVU5XWC2&qid=1309013055&sr=8-2, but all you're interested in is the code after /dp/. In this case, it's B0043M4ZH0.

So, you know that the AZW file with the name B0043M4ZH0_EBOK is The Ultimate Hitchhiker's Guide to the Galaxy.

Well, by going to the "Content Folder" in Kindle "Preferences," and clicking on same, I found the .azw files, but greyed out. They are in ~/Library/Application Support/Kindle/My Kindle Content/.

You can also use the "find" command in Terminal: find ~ -name '*.azw' to locate them.