I think that it has probably always been difficult to define "literature", but today few people would even try. University Eng Lit courses include books, which in times past would have been dismissed as rubbish. Personally, I think that whether a piece of writing is or is not literature, depends less on how it is written, but more on how it is read. Even the most highly regarded piece of literature - eg Tolstoy's "Anna Karenina" or E M Forster's "Howard End" - can be read simply as a story, paying little attention to the quality of the writing, moral situations or characterisation. On the other hand, what might be regarded as trashy thrillers or romances, if read with care and sympathy, can offer insights into the use of language, social background etc. (If it hasn't already happened, I expect "Harry Potter" will soon make his entry into Eng Lit reading lists - and why not? - so long as I don't have to read it.)