I did find a way to do this, although it's not ideal - it requires downloading, uploading, etc. any time changes are made (i.e. if I buy something new and want it to be available to my site.) It also includes items you haven't actually purchased (wish lists.) Right now it's not a huge issue; this was a development exercise for me and I don't particularly care if the information is totally up-to-date now that my database is populated.
Here's what I had to do:
- Install the Amazon Library Exporter extension in Chrome;
- Go to https://kindle.amazon.com/ and click My Books, then All Books;
- On each individual page (can only do one page at a time, not the entire list) click the extension icon in the URL bar;
- In the popup window that comes up, click the Google Books button (you must have a google account);
- After completing steps 3 and 4 for every page of my Amazon books, go to books.google.com and click My Library;
- Your books will have filtered into one of several lists, depending on the status of them in Amazon (wish list items show up under To Read, the others may be under Reading Now or Have Read.);
- Click on each list, and under Options choose Export as XML.
From there, I used an XSLt stylesheet and some PHP code to transform the XML into a SQL file, which I then imported into mySQL via phpMyAdmin.
(It didn't really take as long as it sounds like it would once I figured out all the steps. But it isn't a one-step, click and you're done solution, and as I mentioned, it also is a one-time thing - if you want updates you need to redo all of the steps. I haven't found a way to filter out the books that I already exported from Amazon and only pull in new ones since there's no way to sort the list by date.)