Your smartphone can also be used as an eReader. You might think the screen is too small, but reader apps adjust pretty well and you can always zoom in to enlarge text.
If you’re already an eBook customer and have purchased Kindle or Nook books, you can go ahead and download either the Kindle or Nook apps from the app store for your device.
Any books that you’ve already purchased will be available there for you to read.
Both of these apps are free.

Here’s a look at what the Kindle library looks like.

And here’s the book open.

I can zoom in by pinching and zooming.

iPhones come with the iBooks app pre-installed, You can use iBooks for purchasing and for reading books.

Google offers the Google Play Play Books app which also allows you to buy or read books.
Those are certainly not the only eReader apps available. Search eReader in the app store for your phone to get a look at the other options.
I have been using my phone to read Kindle books for years and it is the only way when you’re in doctor’s offices or airports waiting. I also use it frequently while eating lunch.