The Kindle Unlimited program is $10 a month, and it gives you access to over a million books and comics on Amazon. Amazon is currently offering a deal: 3 months for 99 cents a month. I especially like that you can read tons of comic books and graphic novels this way. I'm currently reading Grant Morrison's The Invisibles on my iPhone. It works really well using the one-panel-at-a-time mode. I prefer reading comics this way over reading the print version.