I just purchased and downloaded these books. They downloaded with the .prc extension. I copied two of the files directly from my download folder to an SD card to load on my Libre Pro--one as-is and one with the extension changed to .mobi (couldn't remember if my reader recognized the prc ext. or not). Both books opened fine and are DRM-free (and at a quick glance seem to be well formatted with nice illustrations).
I had trouble downloading one file this morning and contacted them through their website. I heard back within an hour or so and they had fixed the problem.
for good customer service!