Our ebook market is so fragmented that there's no DRM solution that all (or even most) e-readers work with. If you add in smartphones and car stereos (for audiobooks), the situation gets even worse. Therefore, most publishers use watermarking instead of DRM, usually giving you Epub, PDF and Mobi, which you can read on any device you want.

The most common form of watermarking, at least when epub is concerned, is a 1px by 1px div containing a nonsensical hex or base64-encoded string. There are rumors of watermarks in cover images and even the text itself, though. Apparently, sometimes spaces get removed or extraneous spaces are added, lines get split up a little differently, or some common spelling or typing mistakes are made. Considering the huge number of alteration points you have, let's say 10000 per book, even doing two alterations lets you uniquely watermark 10000^2 copies.

Similar things are done to audiobooks, whether by modifying the audio itself in imperceptible ways, or by modifying the internal structure of the mp3 files. From what I've heard, messing with how frames are laid out and what's in-between them is a common tactic.