once you buy an e book it is yours and you store it in your device's library, it doesn't go anywhere unless you transfer or sync it to another device, even then you have just copied it and still have it on both devcies, e books are cheap because the publishing process involves creating an e book before a hardcopy is printed, to create an e book version for an e reader is just a software conversion.
The only books that disapear are your downloaded library books that expire after 3 weeks or so,