Beyond the good points made by AFLM, "mini laptops" can now be had for about the same price as a Kindle. Their screens are around 9" (diagonal) and offer more room for larger fonts relative to a Kindle.

The drawback is that it would be hard to get most recent (copyrighted) books. Otherwise, there are thousands of classic books that can be found on the web in text format.

A shortcoming of the Kindle is that it is awfully easy to turn a page when you don't want to -- such as when you are first turning it on or moving it around a bit. The keyboard might also be difficult for your father-in-law to use. It's small and the text printed on all of the buttons would probably be impossible for him to read.