^ Kindle Fire isn't below the £100 budget.
The standard Kindle is good and only £69 (ie about the same as 10 new books). It does lack a back light, but that a) means the battery lasts longer b) it doesn't strain your eyes as much.
The Kindle does fit nicely in the hand too, with next/prev page buttons in a useful place.
Buying a device that is dedicated to that one thing usually means that it'll be better at it than one that does lots of things.
EG using a Playstation/Xbox to play CDs will never sound as good as a dedicated CD player (proper hi fi) costing the same or less.
Sure the Kindle Fire (and iPad and Android tablets) will give you an e-reader app, but you'll get a shiny screen giving glare, you'll get less battery life, the backlit screen will strain your eyes and it'll be heavier. Oh and if you use it in public, a tablet is more likely to be stolen.