Hi, and welcome! BookCrossing is about sending your own books out into the world for others to enjoy. Many BCers do trade books; if you're interested in that, see the "Not so Random Acts of BookCrossing Kindness" forum.
While it's OK to register e-books here, it seldom makes sense to either wild-release or trade them, as there's not usually a physical copy that can actually travel. [Exceptions include e-books-on-CD; if you have a public-domain copy, or legitimately-purchased e-book-on-CD, you could register, label, and release that.]
Do read the site's FAQ and "How to BookCross" pages, under the "information" tab. And if you have more questions, just ask!