I've done this before (at least 5 times now) and have always received a reply (on Twitter, email and even on LinkedIn). But the thing is, I don't send appreciation messages. I ask them doubts I have from their work. Irrespective of whether it's fiction or non-fiction, I've felt authors are very ready to reply to you if you ask them specific doubts from their work. More so if you offer them some good suggestion for inclusion in future revisions.