Hmm. OK lets see.....
Some e-Book readers do have a basic web browser and maybe an MP3 player.
A Tablet can run apps, games, play video, music, capture video (even HD), has handwriting recognition for office apps, etc..
The basic functionality of an e-Book reader compared to that of a tablet also means that the price is around $100-200, compared to more than $500 for a tablet.
The e-Ink screen technology with no back-light with extremely low consumption will easily give you a month of battery life on an e-Book reader.
The tablet will run dry after a few hours of use.
If the reading is primary objective, an e-Books eInk provides a superior reading experience. For everything else, there's Tablet.