If you use the Mac App Store version, the location is ~/Library/Containers/com.amazon.Kindle/Data/Library/Application Support/Kindle/My Kindle Content.
In Mac OS Lion (or likely in later OS X versions like Mavericks and Yosemite), the user library is probably hidden.
To access this library and consequently Kindle library itself.
Right click finder icon in dock | go to folder
Type in the following string: ~/Library/Application Support/Kindle/My Kindle Content/
Source: http://www.quora.com/Where-does-Kindle-for-Mac-store-the-e-books-on-my-computer
