I could be wrong, but it looks like they use their own proprietary DRM and you have to use their reader software if you want to read the books you buy offline -
from http://about.ebooks.com/about-ebooks-com/
Proprietary DRM
We have our own DRM system that has secured the content delivered via our Ebook Reader apps since 2007. It is not based on Adobe ACS. Having our own DRM system improves usability for our customers, and enables us to respond quickly to new requirements and trends.
Do I need special software or hardware to read eBooks?
All you need is your PC, laptop or hand held device and the free Reader software. We offer eBooks in three different formats: PDF download, EPUB download and Online Reader. Our Online Reader requires no software other than an internet browser. For downloading, we will provide you with a link to download the appropriate Reader software free of charge when you make a purchase.
I haven't purchased anything from them yet but if you select an O'Reilly book it says: "Read online, or download in DRM-free EPUB or DRM-free PDF format."
Also, on this page[1], it says: "Most of the ebooks available to download from eBooks.com (but not all) are protected by Adobe DRM."
Given the above, I would assume that downloads of O'Reilly books would not be encumbered.
