I used to have very bad handwriting. Now I have very good handwriting. You don't need a book or to buy a special pen or start a new hobby. Just. Take. Your. Time. Writing. Each. Letter. Judge them individually. Is this clear? Is this legible? Is everything the same size? Take your time writing clearly and you will get better, forreal, and you'll get faster and it will become natural.
(I love good handwriting).