Since I've been studying astrophysics, I've noticed that I don't know anything about plasma physics. This prompted me to buy a book on plasma physics, namely: Introduction to Plasma Physics: With Space, Laboratory and Astrophysical Applications by Donald A. Gurnett (Author), Amitava Bhattacharjee (Author).
While trying to decide where to shelve it, I noticed that I already had a couple of books mainly about plasma physics, if by "a couple" you mean 6 or 7.
This suggests a disturbing possibility to me: It may be that owning a book does not mean you know anything about the contents. Who could have imagined?