At first glance you might think, so what? This is just a re-write of the standard fantasy monk. Perhaps it is, but where it scores is that the entire underlying philosophy and history of the Ayutthayan monks is wound through the game mechanics rather than being bolted on as an afterthought to explain what that monk is doing in your fantasy campaign world where there might not really be any of the sort of traditions that underpin a classic oriental unarmed fighting style. (I remember way back in the 1980s playing a D&D Monk as a Chinese person perpetually confused with the standard 'cod-European' fantasy world in which he found himself...)
Here the opening text paints the scene of a single adventurer who retired to a life of contemplation, but was pestered by visitors... some of whom stuck around to become his first disciples, and who - being themselves proponents of different fighting styles - created what became several different strands of the same core martial philosophy. These strands are reflected in the options available as class features as the monk rises in level - the ones you choose chart your progress in your preferred style. Some are acquired by means of mystical tattoos, a beautiful and traditional touch.
The combat styles are based on Thai martial arts and are described well, enabling each monk to develop a coherent - and potentially devastating - combat style. There is also a selection of feats and an array of new weapons appropriate for ayutthayan monks, a new tactical manoeuvre called a Bone Break and a sample character to let you try out this class or just give you some ideas to get you going.
If you want to play a monk, this is a good way to go because of the coherent background philosophy that underpins the mechanics of the styles available. Pick it up, mix it in to your campaign world's history and you do not need to explain how you came to be.