Click on Library.
Select the book to copy. Right click on it.
In the options that appear, select "Show File in Explorer"(Win) or "Show file in Finder"(Mac)
Copy the books from the location and mail it. The book can be a pdf or an epub.
Make sure that your recipient also uses the same Activation ID that you have used for authorizing ADE on your machine. (There is a limit of 6 on the no. of devices that can be activated with the same ID)