TOR says their books are DRM-free, but I just bought three TOR books via Amazon.com and downloaded them from Amazon's "Your Content" page. They came down as .azw files, which Calibre says are DRM-locked. I can't find anything about how to get my DRM-free TOR ebooks in a format I can read on a non-proprietary reader.
EDIT: I recently (early 2022) took another crack at this. I'm not sure what else (if anything) may have changed, but I managed to get my ebooks converted using Calibre and the DeDRM plugin. The two key issues were:
First, I now have an eink kindle and it's set up in my Amazon.com account. I don't actually use it, ever, but it's there and it has a serial number, which I entered into the DeDRM plugin config. I'm not certain this step is absolutely necessary, but it's part of the Calibre set up I used. But this alone wasn't sufficient, I also needed...
Second, I used amazon.com and selected "Download & transfer via USB", and for the device I selected the eink kindle, not my Kindle Fire, which I had selected previously. When I had selected the Kindle Fire, it didn't work. When I copied the eink kindle's serial number into DeDRM, it didn't work. But when I also downloaded on my desktop for transfer via USB and selected the eink kindle as the destination device, that worked.