Beautiful Teams is the first book I came across in the series. Actually I bought it as a gift to my manager (I lost a bet with him; nevertheless an excellent fellow!). I loved the cover, started browsing through the book; eventually the book was so wonderful I ended up reading top to bottom.
The most interesting thing is that you get to read about variety of very different teams something you may never come across personally e.g. team that built software for space missions; their troubles, practices, what worked, what didn't work, etc.
I am reading Beautiful Architecture now. I loved to read about KDE development team. Things that you will never hear about in commercial development e.g. requirements from government organization/ corporates/ or a home user all are of same priority and considered on equal measure; or how architecture gets evolved in an environment that doesn't have corporate hierarchy; or how innovation is more easily possible in a culture where doing-it-for-love-of-it than what-is-the-return-on-investment culture.
You may agree to the opinions or disagree; in fact, many of the opinions are contradicting between chapters; but to me O'Reilly's Beautiful series is simply..hmm..beautiful!