What are the best literary extras?

We all know about the extra features on DVDs: those behind-the-scenes documentaries and deleted scenes that accompany the main feature – often uninformative pap, very occasionally sublime. But what about extra features for books?

The trend towards stirring more and more content into a book first struck me when I read a Harper edition of Tim O’Brien’s If I Die In a Combat Zone some years ago. Its cover promised what it called a P.S. section, a supplementary dose of reviews, interviews and articles about the title in hand. And, sure enough, there they were: one publisher’s attempt to add more value to their product in an uncertain marketplace.

I’ve come across similar attempts since – but, really, the idea of adding written value to books isn’t anything new. Just think of the introductions, by this author or that academic, that have preceded classic works for decades. The question, really, is just how much value is added.