I really liked the Quintessential Monk.  
Granted, it's 3.0 material, but I felt that the feats, the uses for the skill checks, and especially the "Legendary Forms" section were really neat ideas that I know would be a huge pain in the arse to get any DM to approve (especially the forms).
What's great about Quint Monk was that nothing was "obviously" broken (I wonder if anyone would make a case for anything though).  It doesn't have that much "direct combat" splat material in it, so it's definitely not just a "how to make your monk stronger than everyone else".  Though it definitely DOES provide ways of improving on the monk class so that it is more fun to play, and versatile.
The Prestige Classes and the School Style stuff is interesting... but at the end of the day not that useful or even fun.  It's still a good read and can easily inspire what kind of "monk character" you would make based on the archetype the PrC's were using. 
The Legendary Forms was the most "unique" part. It was kind of like a wizard's crafting stuff... only instead of making magic weapons, you gained a "feat".  By spending several in-game months training, and then expending a sum of xp, you'd gain a new ability. The abilities, however, are very subdued.  There's no "fireball" or "increase your BAB" stuff in there.  Some skills are better than others, and make characters pay for the XP as a result, and increase the time span greatly.  And, to top it off... you can't learn more forms than your total wisdom modifier.  
It's a cool book. But... I don't know about Monk II at all. Has anyone ever read it.
PS: Beyond Monks is mostly crunch... it's a good book, though some of the stuff in there is kind of bleh. And... I'm not a big fan of their Martial Artist class, although it's still a quality design.