15 Minutes a Day by Mark Wade
Mark Wade has put together some great books and this one is really cool. When you buy any of his books and Stephen Humphries book "2 Hands, 1 Mind" they work great together and will give you a lot to work on. Mark has put some fun drills together to help you get fast, more accurate and to gain agility. You can change the rhythm of the exercises too and that makes it fun as well. All you need is 15 minutes and you'll grow very quickly.