Do they have a time limit on those books and do you own them (the file) completely once you buy them?
If I am wrong correct me but if you have a kindle or nook from Amazon or Barns and Noble and buy their ebooks you don’t completely own them, they can erase/disable them from your device remotely (I guess).
This happened a year or so back on Amazon. People bought a book written by George Orwell and there was an issue with the publisher and Amazon erased that book from people who had bought it for the kindle.
Here is the link to the article from the NYT that talks about this issue: http://www.nytimes.com/2009/07/18/technology/companies/18amazon.html
There are hack solutions out there which will disable some of the Big Brother features. I am not sure if they hinder Amazon or who ever to disable ebooks bought through their site.
What devices do you use to read the ebooks? I like to have a physical copy of what I read so I print out the book. I know that it defeats the purpose but if someone can disable my ebook remotely or it has an expiration date I would rather have it in a media that can’t be so easily restricted or controlled.