In my experience, most self help books are not worth the paper they are printed on. For medical advice, I would start with sites like Harvard Health, Mayo Clinic, and Cleveland Clinic. These sites consistently stick to with the basics. I.e., lack of focus could be due to hundreds of factors. In order to give yourself the best chance, start out with the basics. (a) are you getting enough sleep? (b) are you eating healthy? (c) are you getting enough physical exercise? (d) how are your relationships? (e) are you financially stressed? (f) are you overwhelmed?

In other words, if you don't take care of yourself and feel you have things under control, no amount of willpower will let you focus.