Without a doubt the best thing you can do for an author, after buying (and reading) the book, is to write a book review.
Don't spend time regurgitating contents of a book. Instead consider the following points:

1. Did you learn something?

2. Was the writing clear?

3. Would you recommend the book to others?

4. Would you read other books by this author?

5. Was there another particular reason why you liked the book?

A book review should not be a place where you spew venom. Remember even if you didn't care for the book, someone else might be looking for the very information it contains.

Your reviews don't need to be more than two sentences long, or one.

Questions?
I am delighted to talk to you about this topic.

Best,

Pauline