I was thinking of the Kindle Fire (seems they offer a free 1 year subscription) but there are quite a lot of bad reviews on Amazon.
My son is always complaining that he has nothing to read (and it is true that he has read all the physical kids books we have many times). I want to get something where he can download age appropriate books for him to read at night (but either offering a free or cheap subscription service - Amazon do one for £1.99 per month if you have Prime and the first year is free if you buy the Kindle Fire as far as I can tell).