I would say that, for me, volumns one and four are the two most practical and valuable for my uses. The middle two have some really good stuff in them as well, but quite a few chapters are pretty focused on specific areas in which I am not too interested. It's good stuff and well done, but not what I'm after. That said, the each have some valuable information as well. But, if you are limited, I'd say pickup volumes one and four.