@naugeleh I would recommend the Kobo store as well. It's owned by Rakuten, which bought the leftover IP that used to be the Borders book company. They make very good readers, and have great support across the world, and seem to actually like the eBooks.
Stay far, far away from Barnes and Noble -- the company is antagonistic against eBooks today, even though they still make hardware and produce apps.
Also, I'd stay away from the Apple store for eBooks, since you have to have an iOS device.