"Truth in Comedy" is a decent little book that lays out well the philosophy of Improv and the Harold structure. "Art by Committee" reiterates some of what's in Truth, very slightly expands on some of the concepts, but mostly it's just chatter about IO, improv, and Del Close. There's a whole lot of former students talking about how great improv is. There's a whole lot of vague ruminations on how great improv is. And there's a whole lot of pretty pointless... addenduming (if that's a word). The book is like one big addendum to Truth in Comedy, one big "what I was saying is..." and "oh yeah, I forgot to mention this (insignificant thing related to something from Truth)". All of this is stuff you'll hear from your teachers at any decent improv class (minus the personal stories, of course). Plus, it's just not that well written. It's poorly organized and poorly conceived and the writing style is far too conversational. But given that not much is being said and really, it's all just fluff and filler, I guess an overly conversational style is the only way to pad it out.
If you live somewhere with no access to decent improv then the DVD may be of some help. The book... well, if you're in Podunk, Nowhere the book might help flesh out a few things from Truth in Comedy as it has some examples of things that worked out or didn't and how they might have gone the other way had the players followed/not followed the rules, stuff like that, but for anyone else actively studying improv, don't bother.