From The Ground Up was the first book of his I read. Loved it, printed it out, and I still flip through it sometimes. It touched on a bunch of different training methods, programs, theories, and techniques. His Olympic lifting background really comes through, maybe more on this than in any of his other stuff, but that’s not necessarily a bad thing.
To Grad from Dad I also really liked. It’s Dan writing “notes on life” to his daughter, so there’s not really anything about training, but there’s plenty of good information and his thoughts on pretty much everything else.
Never Let Go is essentially a collection of a bunch of his articles from here on T-Nation. That’s not a negative at all. Very nice to have all in one place, but if you’ve followed his writing for a while and read his older stuff, you probably won’t see anything new, though it’s always good to re-read. New insights have a way of popping out of something you’ve read 11 times before.
I hear nothing but good reviews of Intervention, but haven’t read it myself so I can’t comment on it.