Burning a detested or feared book is a perfectly good speech practice--IF YOU OWN THAT PHYSICAL BOOK YOU'RE BURNING! In that case you're involved in a symbolic act, which is protected speech, much like hanging Guy Fawkes or Hitler in effigy.

But if you are trying to eliminate the presence of the book in the public library or the school library you are trying to effectively destroy the thoughts represented by the book, by preventing their transmission to the population that uses that library. And of course literally "burning" is not necessary.

Dave Oesterreich