In my case, I think it's a book from James Clear "Atomic Habits". Because after this book I could realize the importance of consistency and habits to accomplish long term goals. You can be a person with lack of motivation, it's literally impossible to be motivated on a daily basis. We all have some troubles in our relationships, work, some rough days. But if you'll create a habit to do something useful, like going to the gym or learning English, an example, you'll be able to do it without breaks in these days and without using willpower so much, like brushing your teeth. As James Clear said: "Every action you take is a vote for the type of person you wish to become" and it's a good attitude to life, at least in my view.