Focus is a skill that you develop rather than a technique that you exercise.. The main reason people struggle to focus is that they don't understand what focus really is. Most of us think of focus as willpower, the effort you put into keeping your mind focused on one thing when the mind wants to go somewhere else. But that's only a small part of focus and by far the least.
This book shares a lot of highly tactical processes that you can use to become more effective in your daily life. What it lacks is a lot of the real world context that comes from a lot of the books that these tactics came from. Very much recommend this as a reference, but I would also recommend a lot of the source material be read first.