This KOBOLD GUIDE was the first in the series that I had ever read, and I had high expectations. One interesting detail I noted right away is that famed DM Matt Mercer is clearly the inspiration for the cover, but he doesn't have an essay featured within.
Not a big deal, but funny nonetheless.
The book is sorted into different sections, grouped into themes such as Before the Game, During the Game, etc. Each section contained a handful of essays written by a variety of GMs, with quite a broad range of experience and expertise.
Some of them are very storied personalities, involved in very key games, developments, or events. Others are... well, just people with a bit of web presence or relatively minor impact. Not that someone's advice should be judged solely on their previous work, but I feel it should at least be mentioned that not every author will necessarily come off as an authority.
To give a bit of context, I've been DM-ing weekly for about 15 years, now, with a variety of groups, people and systems. So while I haven't been playing since the very beginning, I have been around since AD&D, and have a considerable amount of experience in the GM's chair.
That being said, I naturally found a lot of the information in this book redundant. Things like dealing with shy players, allowing players to shine, or improvising sessions. I've come to those conclusions on my own, over the years, and found that a lot of the information corroborated my own experiences. This is not necessarily a criticism, however, since it is always reassuring to read 'advice' that you already follow, in the sense that it gives you the feeling that you are on the right track.
Others might also have very different experiences. I have dealt with many new and shy players, so naturally I've learned how to cope with that. A GM who has not might find these sections more useful, even if they are otherwise just as experienced as myself.
There was also certainly some stuff in here that I found new and interesting, and will be incorporating into new campaigns.
In conclusion, this book really feels like a series of panels given at any given convention, market, or other event. Some of the panelists will be legendary, others will be unknown. Some of the panels will be very helpful and interesting, others will be relatively banal or elementary.
You will learn something new, and you will read some stuff you find obvious, or flat-out patronizing. However, if you go through the book, you will come out with an expanded mind, and some new ideas. For that, I found this a worthwhile read for any GM.