The best investment you could ever do is for yourself. Personal growth in character, attitude, mindset, and skills can be had by reading personal development books. There are all kinds of self-help books. So, depending on which area of personal growth you want to focus on, there is no excuse for not doing so. As you might have noticed while looking for personal development books, most focus on attitude. One of the reasons why is because it is our attitude that motivates us to take action. Without the right attitude, then there is no willingness to learn. As such, how could one then become a better version of himself/herself? But with the right attitude, not only do we seek knowledge, we also persevere. In anything we do, there are trials and challenges. How well we cope is dependent on how much we have grown ourselves.