It's everyone from Amazon to Apple iTunes. There are probably about 30 different ones, and we use a commercial distributor that basically sends the file to all of them. In some ways, the copyright protection is on them.
On the ebooks that we sell off our own site, we have a limited digital protection, and if we send out desk copies to academics for review, we ask them not to copy them.
Basically, if an individual checks a book out of the library and decides to copy it for private use, that's fine.