Mike wants to use his Kindle to read comic books. Leo says there's a far better option, and it's owned by Amazon as well. It's called ComiXology. It's designed exactly for reading comics, while the Kindle is really made for the written world.
ComiXology will enable him to zoom in and out, and it also works with all comics. He can use it on his laptop too. The Marvel Unlimited app has an all you can eat monthly subscription, so if he's a Marvel fan, it's the way to go.