Is there an eReader application that runs under Windows, Android and Linux flavors? Someone just gave me a tablet but I also use MS Windows and Linux Mint. I could wade into searching online but I presume most work under only one OS.
This probably is not the answer you are looking for, but I highly recommend using Google Play Books for cross-platform reading. It renders things so well that it's my default choice for everything.
I'm not criticizing FBReader or other readers which are cross platform, but many of these apps just don't render ebooks well. I have basically given up trying to format for them because they override so many publisher defaults. About the only epub reading app that doesn't override defaults is Adobe Digital Editions, but there's no Linux edition version -- though I suppose you could use WINE or something like that. (Curiously Adobe DE has a mobile android version which works pretty well though has minimal features).
There's a Readium plugin in Chrome that lets you view epub files offline in any Chrome browser.
Calibre is cross-platform too, though the reader is not particularly reader-friendly; it was built mainly to let you see the result of converted ebooks.