You seem to have to email things directly rather than attach them to a separate email, as you do in Windows.
But there is a compatibility problem between Kindle books and other e-books. I think the way round is that you to convert kindle books to a different format for any other ebook reader to access them. And doing that seemed very complicated when I read up about it.
If you have a kindle app too, one way is to make a 'joint' amazon account that you can both access and put any kindle books you buy on it, so you can each share them. You have to trust the person of course, since they could buy stuff and leave you paying for it. I remember this being suggested when I asked how I could share my kindle books with my mum, as though there was no simpler way.