Which ebook app are you using? Are the books DRM "protected"?
A book with no DRM is trivial: copy the file to another device and just read it in whatever app you choose. With DRM, I dunno: maybe you can just copy it and read it, provided you use the DRM-holder's app and are signed into that, or maybe you have to download it again from the DRM holder (I can imagine Amazon working that way). So this may be an important question.
The reason I don't know what DRM does to this is that where I live it's legal for me to remove that **** from a book provided it's just for personal use and I don't redistribute the file, So it's legal for Amazon or whoever to sell books with DRM, but they have no legal right to prevent me from removing it in order to read the book on a device of my choice). So I don't know how DRM affects this equation because if I ever do buy a DRM-protected book the first thing I do is remove the DRM, as a matter of principle. But I don't know whether it's legal for you to do the same, because I'm not in the USA.