If you're buying through Amazon and your tablet supports the Kindle App, you're going to be able to read books on both your laptop and your tablet. Other vendors offer PDF and ePub titles using Adobe's DRM scheme. You can use those titles on multiple devices too, provided that they're registered with the same Adobe account. (It is possible to use Adobe's DRM without registration, but you can only use those books on one device.) Many apps support registration directly, but some devices must be registered via ADE on your computer. Of course, it is best to buy DRM free if possible. If you're planning to keep the book for a long time, it is probably worth paying a little extra to do so. (After all, who knows where these companies will be in 10 years?) Oh, on point 3, most DRMed ebooks are encrypted so I doubt that backing up the file will have any effect on the license agreement.