Writing a mystery is a challenge for any writer. Granted, there is an infinite number of crimes to work with and, if the writer creates an interesting character(s) to be the crime solver, it looks like an easy task to develop a series. Just change the crime and you've got your series. Sounds easy, but the more challenging aspect of writing a mystery is handling serendipitous events. For example, how is it that the crime solver just happens to be in the neighborhood or meets someone who has heard about a crime when the crime occurs? Serendipity is the "aptitude for making desirable discoveries by accident". There are serendipitous discoveries, clues that appear when least expected, serendipitous meetings, serendipitous events, and more to contend with. The challenge is to make it all seamless, that all of the placement and discoveries of clues, including the appearances of red herrings (clues that don't fit) are presented in such a way as to guide the reader down the path the writer intends. This takes a special kind of writing skill, i.e. managing many serendipitous events/discoveries while progressively creating tension and reaching a sensible climax. One way to do this is to constantly be aware of the main point. Alfred Hitchcock coined the word "McGuffin". To create interest Hitchcock used this device in his movies. It is a device or plot element that catches the attention or drives the plot, something introduced to hook the viewer. Effective mystery writers evaluate each scene and each meeting of characters for their consistent relevance to the main plot.  Readers do not appreciate long, drawn out, meaningless exposition or long, unrelated discourses to plod through nor do they appreciate foolishly placed serendipitous events/clues. For more on this go to www.writefromhome.com/writingtradearticles/317.htm . Look in the archives for my "Serendipity: A Writer's Bane or Blessing.