Okay, so Kindle For Mac is an okay program and I wanted to explore it’s reader capabilities a bit more since Calibre’s built in reader is a bit lacking in aesthetic (important when reading for long tracts) and slow to respond to user input. However I discovered something very annoying when first loading up Kindle; it only imports the books that you have purchased from Amazon, not the Send-To-Kindle documents that are also on Amazon Cloud Drive. Very disappointing.
There is however, a workaround! If you go to the Kindle menu, then preferences; the section headed “Content Folder” shows you the folder that Kindle For Mac reads in books from. Unfortunately this is some hidden/esoteric file that is more of a pain to get to regularly, so my solution is to create a “Kindle Books” folder in Documents, then click Change in the Kindle For Mac preferences and set it to the newly created folder.
Dump all the ebooks that you wanted to appear in Kindle For Mac into this folder and restart the application. They should now be available to read.