There is a need to be careful of the term "open source" which is usually about being able to base your work on its. I suspect what you're looking for is no digital rights management as you have already mentioned. But, a lack of DRM, or a free book, does not imply in any way that its open source. Likewise readers and management tools. Calibre is free, but its not open source. I doubt that any of the ebooks are open source... free, yes. Open source, no.
I'm surprised nobody has mentioned Phoenix Pick. They have a pay-what-you-want book once a month. It used to be free but theres an option to pay if you wish, or nothing if you don't. You sign up for a newsletter which only comes monthly, alerting new books available and the monthly book.
http://www.phoenixpick.com