I can't offer book writing advice. I too have a short attention span so what I do is have to set aside a time for certain activities. At work, that's not a problem. But when I need to study for a quiz for the class I'm taking, I set aside two evenings (Wed. & Thurs.) to study. For me, it's all about setting aside time for specific activities.

If this doesn't work for you, you may have ADHD and they have medication for that. See your doctor.