I posted a tutorial on DRMed ebooks on the EZ Reader (Mobiebook) Forums, see
Secure MobiPocket ebooks.
If you use Windows MobiPocket Reader to manage your secure ebooks, it "knows" where you bought ebooks from and can update them for a new device. However, I much prefer to manage this myself. Also, Windows MobiPocket Reader does not "do" subdirectories - which are a significant feature of ebook management on the EZ Reader.
Any Secure ebook format has potential problems with "permanence". One advantage of MOBI is that it isn't actually secure, because the python script mobidedrm can strip the DRM providing you know your device PID. Actually striping the DRM, even for personel use on ebooks you paid for, isn't thought to be legal by most commentators in the US - although this has never been tested in court and there are counter arguments (e.g. explicit exceptions for some ebooks). At a minimum, the existence of mobidedrm means that you don't have to worry about an ebook vendor going out of business.