TLDR; 
 100 hands is a collection of 100 unbelievable thorough analyzes of online poker 6-max hands, played by coaching students or the author himself. It is without any doubt among the best poker books for beginners as well as more experienced poker players, who are ready to put some work into improving their game. 

The idea behind the book is similar to your exercise books back in school, so it lets you make your own analyzes first, and then see how close (or more likely how far) you got to the master afterwards. Repeat that 100 times, and you will inevitably improve as a poker player. 

 You can read it on its own, and learn how Peter 'Carroters' Clarke thinks about 6-max poker. However, the book only, really starts to shine, when you first read 'The Grinders Manual', and then use 100 hands as a repetition of the insights and techniques acquired from there. 

Unlike Clarke's insanely good 6-max poker book The Grinders Manual, which is extremely well-structured and methodological in its approach, there is no particular order in which the different poker problems are presented in 100 hands. This is actually by deliberate choice, as it intends to simulate a real poker session, where you do not (often) have 10 consecutive hands, where the core problem is the same.

Every hand is different. The stack sizes may be different, the opponent may be different, you position is different etc. In some hands, you must make use of your gained knowledge of polarized vs linear ranges, in others you consider if you should play GTO or exploitative etc. Every hand in a session is a new problem, and the same is true for this book. So if The Grinders Manual is your well organized study book, where you can pick one topic at the time, then 100 hands is the interdisciplinary training book, which really separates the sharks from the fish.

The level of detail that is handled in each of the 100 hands is impressive. Having proudly worked out a complete range analyses for a hand, only to find out you missed some other important information is a mixed experience. It is always impressive to see how the pros use all the relevant factors for a hand - and just as importantly how they effortlessly discard the unimportant factors from the train of thought.

You may think that the relevant point in a hand is whether villain is a regular or a recreational, only to find out that the determining factor in a hand is the bet size. Learn to focus only on what actually matters, and your decisions will be way easier.

Wish that was true, but it isn't. However, establishing solid off table practice routines will provide you with a confidence that your plays are correct, even when your downswing says otherwise. One of the worst psychological effects of variance and downswings is that you start doubting your game, and this again leads you to make more and bigger mistakes. Knowing and having proved during hand analyses after hand analyses, that your game is sound can prevent from going quickly from bad to worse.

You may have read that in order to become world-class in any field, you have to spend 10000 hours of deliberate practice on that field. This book alone, will not get all the way there, but it will definitely ensure, that you get a good head start, and set you off in the right direction. Some (most actually) give up even before they start, which makes it so much more profitable for the rest of us, who are ready to go all the way. The time spent on improving your online poker skills is building up a nest egg. With online poker however, you will start reaping the massive benefits way before you hit the 10000 hours mark.

 The Chinese philosopher Confucius said that: "A journey of a thousand miles begins with a single step". Take your first step towards your poker stardom here. 

If you're looking to buy just any 6-max poker book, to help you improve in especially online poker 6-max cash games, then I'd strongly suggest the remarkable The Grinders Manual as your first choice. Once you have been through that book, have understood all the concepts introduced in it, then you are ready to get the full potential of 100 hands. The book utilizes concepts explained in full, priceless details in The Grinders Manual.

Unless you already have classified insider knowledge of how Peter Clarke thinks about online 6-max poker, then 100 hands on its own will still impress you - but you will probably not get all the outcome from it, that you would have the other way around.