We tend to assume that plagiarism is a reasonably modern day phenomena but a fascinating article from Paul Collins "Dead Plagiarists Society" illustrates that is not always the case. In fact it would appear that Victorian authors stole work from earlier authors who in turn had stolen it from even earlier ones, and the article gives some nice examples of this.
More importantly, it provides a fascinating use for Google Book Search, which I'd certainly not considered before, and it's actually a rather interesting experiment - take a partial sentence, plug it into book search and add words onto the end, and very quickly you arrive at a short number of results, allowing researchers to get a very clear view of the 'history' of a small passage of text. This in turn could then be expanded to provide an insight into a particular authors reading and research habits. Consequently we can all start to play literary detective quickly and easily; now where did I put my Sherlock Holmes?