Pick the one that gets the job started and adjust the purpose as you learn more.
Normally I want to take the shortest route to my destination. However if I have to buy milk along the way I'm going to make a detour. To meet my purpose of reaching my destination and getting milk along the way. So I plan.
That's the point of having a purpose. If you "can't seem to come up with what I really want" It means you're not interested, not curious and perhaps have no need for the book. The question then is... why are you spending your precious time with the book?