You should also get a notification of any replies, I believe it's the default setting of these bulleting boards.
I just wanted to mention that an ebook
is just the BOOK. The device you own is called an ebook reader
Yes yes, I know, nitpicking ... but it might lead to confusion further on.
As to your original question: like kennyc said, if the books are "open" and not copy protected, you can drag the files from your device onto your desktop computer (or laptop, obviously) and then drag them onto your wife's reader.
If you bought the books, chances are pretty high that they are DRM (Digital Rights Management) protected and won't open on another device. In that case you might want to look into removing that copy protection, there are a number of threads on this subject. But since it's a rather shady thing to do, depending on what country you are in, concrete tutorials of removal won't be found here.