Smashwords is a great source of DRM-free ebooks. And also a great source for some books that might never get published by a major company.
You can find some interesting and unexpected things coming from Smashwords. For example, Doug Gelbert has produced a large number of brief "Look Up!" guides for walking tours in American cities.
https://store.kobobooks.com/en-US/eb...city-midtown-1
https://store.kobobooks.com/en-US/eb...-hill-district
https://store.kobobooks.com/en-US/eb...loop-south-end
There are a LOT of these, all priced at $0.99. And they're not all major tourist destinations; this one brings me close to home:
https://store.kobobooks.com/en-US/eb...g-pennsylvania
Doug Gelbert has also produced a series of "A Bark in the Park" books about good places to hike with a dog. At the Kobo store some of those are $0.99, some are going for $2.99.
And in keeping with this thread, they're all EPUB2 (DRM-Free)!