I just looked up my own O'Reilly book on ebooks.com.
http://www.ebooks.com/95729334/designing-data-intensive-appl...
I can't see anything about DRM on the page, but under "supported devices" it says "e-readers with Adobe Digital Editions installed". Isn't that DRM enforcement software? Could you clarify please?
Edit: this page explicitly says that readers need to be compatible with Adobe DRM https://support.ebooks.com/hc/en-gb/articles/214119286-Guide...