KOReader, though most people will probably find the UI off-putting at first.

Thing is, it's perfect for use on an e-ink device, which is what's it's primarily designed for. The desktop (Linux and Mac) and Android versions are just icing on the cake, and they all work the same and can sync reading activity between them. Tons of features, options for tweaking book layouts, plugins for integration with other services, some integration with Calibre, etc. It takes the "kitchen sink" approach and I love it. I've found spending the time to learn it to be really rewarding.