1. Leave a review on Goodreads (or anywhere else).
It doesn't have to be long. You can even just say, "I loved it!"
2. Follow them on social media.
And for a bonus point say hi and let them know you really enjoyed their book. Never underestimate the power of casual encouragement. That author might receive your note on a day they decide to stop writing. 
3. Do a cool Instragram pic of the book!
Say how it gave you all the good feels. Don't forget your hashtags ;)




4. Tell a friend!
Yes, word of mouth is still a thing.
5. Ask your local library to order it in! 
This is awesome because it helps other people get access to the book as well. And did you know that authors get a small revenue when their books are in libraries? Yes, it's true. So order away, friends!
This is the public library in my home town of Halifax. Isn't it gorgeous!