If you have more to do than you can possibly keep track of, then I recommend reading Getting Things Done: The Art of Stress-Free Productivity, by David Allen. It is available in both paperback and Kindle editions.
Without question, this has been the single best book I have read in my adult life. The principles it contains help me focus every day. And while the approach definitely isn't for everyone, it contains simple, practical, and easy-to-implement strategies that anyone can incorporate without abandoning a trusted system that works. I didn't have a trusted system when I began this book, but I do now.