If you sell through a publisher or indie publish via an ebookstore, you can usually choose to apply
DRM to encrypt it.
Speaking purely as a reader, however, I strongly disapprove of DRM, and would like to point out it is very easy to break any DRM no matter what; see here:
Copy Protection >> Technical Challenges, here:
Digital Rights Management >> Shortcomings, and here:
Analog Hole. If you have sold the book to someone, you cannot truly stop them from doing whatever they want with it unless you stop them from reading it.
DRM is a
severe nuisance to the reader who bought the book honestly, preventing him/her from reading it across multiple devices for example; meanwhile, anyone who truly wished to distribute the book to all and sundry will not even be slowed down (assuming they get their hands on the book in the first place, which requires them to buy a copy first
).
On a more optimistic note, Baen Books has never used DRM, and
Tor Books went DRM-Free 2 years ago. Both maintain that there is no discernible increase in piracy as a result, and sales have not been affected.