The answer is maybe. It depends on whether the ebook has digital rights
assigned to it by the publisher, distributor or author.
You can tell whether digital rights have been assigned. Open ADE in
Library mode. Then, hover your cursor over the small arrow next to the
ebook's name. A drop-down menu will appear. Pick 'Item Info' from that
menu and click on it. The information on digital rights is in the middle
of the screen. If it says 'Copying not allowed', then the ebook cannot be
shared. If not, then you can pass a copy of the ebook to others.
Actually what Frustated just said is wrong (Copying Allowed is for copy paste of text not sharing of books).
The answer is if the book has DRM (ie you got it through .acsm file) then the book is tied to your AdobeID, and is not shareable to anyone else's AdobeID.
If you want to share a purchased book you can either:
1.) Purchase DRM Free books
2.) Purchase from either Amazon or Barnes & Noble - which allow a limited lending (to single person for 14 days - once only per book).
OOPS! Thanks, Jim, for the correction.