Only if it is not copy protected. The iBooks app will let you email PDF files, though not ePub (even the public domain ones).
Very few purchased books are DRM free, and even few of those are public domain. One you can not share. The other it would be stealing even if you can (unless specifically allowed by the publisher).
Amazon offeres you the ability to loan (some) books to another Kindle user for a few weeks. So if you use the Kindle app, you may (depending on the book) be able to loan it to a friend also using a Kindle. I think (but do not know for sure) that this will work with the Kindle app on the iPad.
BTW, discussion of piracy of apps, books, music, or any illegal activity is prohibited on this forum. By talking about purchased books and jailbreaking you are perilously close to the topic.