Did you get the memo?

There are days,  at the library, that we have time to speculate on some of the bigger questions like:
Who sends the memo!?
A book sits on the shelf for months.  One day three people, with no connection to each other, come in and ask for that book.  Who said what to bring that particular book to the general attention of the reading public?  We always check the Sunday paper - maybe there was a review or reference?  A passing comment on Oprah?  GMA? 
Or within weeks of each other, two (or more) authors will release a book featuring a similar story line.  I recently read new books by Dick Francis and David Rosenfelt.  Both story lines involved an Iraq veteran who had lost a leg below the knee.  What are the chances!?