I know you can get Kindle, Mobipocket, and EPub readers for most common OSs (since you don't specify, I'm guessing Windows? you can get them for Windows), and she says they're the unlocked form of those formats. Therefore: yes. I've used several EPub and Mobipocket readers on Windows, Windows Mobile, Linux, and Android, and unlocked books do in fact work across multiple systems just fine.

In the absence of other constraints, pick EPub format (most modern one, widely supported).