When you buy a book, you expect to have it forever — not until technology changes or the bookseller takes it away, as is the case for most e-books sold today.
You can add new volumes to your permanent library if you buy e-books unencumbered by digital rights management, but your choices are limited.
The Amazon trap and the DRM quandary
I wish someone would do to Amazon what Amazon did to Apple.
Let me explain.
Until 2007, almost all legally sold, downloadable music came with digital rights management (DRM). The company you bought your songs from (for most people, Apple) controlled which, and how many, devices you could play it on. They could even take away your right to play it.
You could find sites that offered DRM-free, play-on-anything .mp3s, but their selections were either pirated or, if perfectly legal, limited to obscure recordings of little-known artists.
That all changed when Amazon convinced the major record companies to let them sell just about everything in the .mp3 format. At least for music, the limitations of DRM became a thing of the past — and Apple gave up DRM on music sold at the iTunes store.