Sometimes (most of the time) I just don't know what I don't know, and I find it difficult to set an outcome. I basically pick up the book because the general topic (be it psychology or philosophy or whatever) appeals to me, and I want to get as much information out of the book as possible.
What outcome do I set?
Did you ever remember reading a book that really captivated you to 'want' to read on? This drive to want to forge ahead was in my opinion the result of some 'purpose' you may have set consciously or unconsciously.
I guess the same can be said when PR, your purpose should really evoke some emotion to want to read. IE ... If you want to increase your vocabulary by PR the dictionary ... then say "My purpose to PR the dictionary is to increase my vocabulary so that I can become a better reader" or something to the liking.
When I state my purpose ... it must contain 'passion' ... a wanting to read the material that evokes an emotional response from within!
This can be hard for some type of material, but if you have the book in your possesion, you must have a 'purpose' for reading it.