Your question is largely device-independent. The only way to gain ownership over your purchases (which in the digital age are just user licences, not actual goods that belong to you) is to strip them of their DRM and keep a copy of the file on your hard drive. Then you are free to read and use them however you like, and no one can ever take them away.
It's fairly easy to do this with books purchased from all major retailers that use Adobe copy protection, as well as Amazon Kindle books. It is much harder to crack Apple iBooks purchases, so I recommend you stay away from those.