Derivative works represent a newly contentious area, particularly in defining the level of innovation or fresh creation. Warner Bros, Scholastic and J.K. Rowling are pursuing a case in New York against the publisher of a proposed Harry Potter Lexicon.

The Lexicon is based on an extant fan website that Rowling welcomed because, although it comprised lists of her spells and curses and other material drawn from her Harry Potter series of novels, it sought no commercial gain and may even have played a part in stimulating interest in her books.

However, once that same material - copied but rearranged and in places rewritten - was to be packaged and sold as a book, not only in the US but in other countries where the publisher had sold translation or reprint rights, the resulting work constituted, as most observers read the case, not fair use - where's the added creativity or innovation? - but rather an infringement of copyright and a denial of income from her creation that Rowling was entitled to.

There are other companions and encyclopaedias, for example to the work of Philip Pullman and Terry Pratchett, but these tend to be produced by the original publishers, or even co-authored by the original author, and therefore do not inhibit the commercial exploitation of the original copyright source by its creator.

Other derivative works that have been challenged using copyright have tended to be much more creative responses to existing works rather than so nakedly parasitical.

Lo's Diary comprises a retelling of Vladimir Nabokov's Lolita from the point of view of the girl, not the man. In this case, an out-of-court settlement was reached with the Nabokov estate, as also happened in the case of The Wind Done Gone, published by Houghton Mifflin, and Margaret Mitchell's estate, which was defending the interests of Gone with the Wind.

These settlements underline the commercial nature of copyright and the interests of estates. Many commentators believe that the well-publicised hesitations of Dimitri Nabokov to destroy, according to his father's wishes, the unfinished, unpublished work The Original of Laura are no more than an attempt to create interest in the book ahead of its eventual publication."