I concur. I tell all my friends/acquaintances/people who ask about the series. I suggest that if you can only afford one, get Vol I. If you can afford two, get Vol I and IV. The other two are neat and full of fun stuff, but are more focused and specific regarding styles and skills. I and IV are more general in scope, skill, and technique. They are great for learning how to get it done.
There are lots of other good books around, all useful and pleasant to read, as well as a host of forums, YouTube videos, tutorials, blogs, build alongs, and such forth, many by member from this fine page. But for an all around on hand reference to dog ear and richly absorb, nothing beats the TBB Vols I and IV.