Under DRM, you are not buying a copy but a license, a license subject to whatever restrictions the company empowered to issue that license would like to set. I exaggerated a bit up the page: It’s possible you do own some of the digital books you’ve previously bought because some titles are sold without DRM protections, but my hunch is you have no idea which ones those are and probably didn’t pay any attention one way or another at the time of purchase.