An analogy occurs to me, possibly an erroneous one (not to mention that analogies are usually inadequate presentations of an idea). Consider a restaurant. When you enter to have a meal, do you order everything on the menu? All the appetizers, all the entrees, all the desserts, etc.? Indeed not: we have a purpose (to satisfy our hunger, to avoid things that may be unhealthy for us, to try something we don't ordinarily eat). We order according to our purpose. We can go back to the restaurant later to try something else, with a slightly different purpose. With a book, of course, you can just be hungry right away again, i.e. you can change your purpose and continue on. But we never assume that we enter the restaurant expecting to eat everything all at once. We have a purpose, and we assimilate accordingly.
Another nice thing about post-processing books we've PRed is that it is so much more relaxing than trying to use books in the normal method.
