Get a kindle. There are cheaper models than the Paperwhite, and you can also get discounted ones from woot.com (amazon clearance). Once you have a kindle, you can get kindle deals, which has saved me a ton on novels (I admit I'm not super picky about muh casual reading) but also you can get Libby (formerly overdrive) which uses your Library account to allow you to digital books for free. Cheapest way to the best, most on-demand source IMO.

You can do the same basically for free by downloading the kindle app onto a smartphone or tablet, but if you're a serious reader, that's tough on your eyes.