Kobold Guides tend to be collections of essays written on a specific theme for 5E Dungeons and Dragons. This one, per its title, was to focus on monsters. And, for the most part, the fifteen essays therein tended to stay on topic. A few veered off in talking about roleplaying encounters, pacing encounters and the like - general GM topics. For me, there were a few good tidbits and reminders to take into account. However, I believe this guide is more suited for a new gamemaster who is just starting out with the game and not someone who has some years behind their belt in the GM chair.
While a lot of it might be rehashed advice to beginner gamemasters, this does contain a lot of solid material for running monsters/adversaries, combat, encounters and background. I personally prefer having a physical book to read rather than checking stuff or asking around on the internet, and something like this would have been perfect back in the halcyon days of yore ('90s) when I started roleplaying and even tentatively running games.
Just because it gets repeated more often in the modern online world, does not mean the information is any less valuable.
This is a good collection of essays about monsters; how to use them for combat and storytelling, and also how to build them. Even as a seasoned DM, it gives good ideas and refreshers on how we play the game.
I have other Kobold "Open Design" books and this one is on par with those: a series of essays espousing approaches to Monster use & design. On the whole it's fairly good advice for GMs without a lot of experience designing new monsters, and/or needing help in being descriptive in their monster use.
Like the others in the "Kobold Guide" series, Monsters is packed with usable information relayed by RPG luminaries. If you ever wanted your table's next encounter to be more than "just another monster," this book is for you.
It's a nice collection of tips and tricks from a lot of big names in the form of essays. Giving you small samples of their experience in a digestible way. An easy read if you are looking to elevate your combat encounters, make prepping eassier and better utilize your Monster Manual resources.