Exalted has a large amount of supplemental material. If you want to play Solar Exalted using the Core book and you can only afford one supplement, though, get this one.

The Glories to the Most High series has useful setting information and great Charms to expand on what the Core book has to offer. It's easily among the best material written for Exalted's second edition, and I've gotten a lot of use out of it. It's also cheap!

Scroll of the Monk came out shortly after I had bought the Exalted Second Edition corebook. I was quite excited over it, as it promised to give the players of my nascent campaign a wide variety of stylish combat builds. I was excited enough to purchase both the PDF, from DTRPG, and the physical book via Amazon.

The book turned out to be an unmitigated disaster. Many of the Celestial Martial Arts Styles presented obvious problems even to our untrained eyes (we were new to Exalted, but have played a variety of other games for a long time). Some effects were excessively weak, others too strong, and yet others barely interacted with the rules at all, or else were grievously broken. The book does not have "mild problems" that can be houseruled away; most of it is a complete and utter disaster, easier to rewrite from scratch than to patch.

Do yourself a favor, and do not buy this book. It's not worth your money. Attempting to use it will cause you headaches and bring grief to your game. This is possibly the worst single roleplaying manual that I own, and I own plenty.