From experience, I can tell you that there is nothing more encouraging to an author to receive feedback, both positive and negative. That was one of the things that (a) kept me writing as long as I did and (b) dictated the types of stories I would write.

So, if you come across an author you want to encourage, I highly recommend that you send along some comments. Not only will this repay the author for taking the time to write the story, but it will often have the effect of "inspiring" the author to write more.