learn-c-the-hard-way was created today on Stack Overflow and is being applied to older posts. Is this tag a helpful tag and should it be applied to older posts?
I'm not in favor of this use of tags.
Yes, we do have a few book tags like sicp. But these are notable books. Unless you can demonstrate that "Learn C the Hard Way" is in the same league as, say, Kernighan and Ritchie, allowing such tags opens the door for a tag for every possible book title, and the shoddy questions that follow ("What does this sentence in this book mean?").
Tags are not designed for this. Tags are meant to categorize questions, not to identify specific artifacts like book titles, motherboard part numbers, or even specific software versions except under certain narrow circumstances.
If you really have a question about something in a particular book like "Learn You a Haskell," just put the book title in the question.