Android design guidelines
App design is a vast topic. It is a topic that could only begin to be taught in a book of its own. Also, like programming, you can only start to get good at app design with constant practice, review, and improvement.
So, what exactly do I mean by design? I am talking about where you put the widgets on the screen, which widgets, what color should they be, how big should they be, how to transition between screens, the best way to scroll a page, when and which animation interpolators to use, what screens your app should be divided into, and much more besides.
This book will hopefully leave you well qualified to be able to implement all your choices for the above questions. It unfortunately doesn't have the space, and the author probably doesn't have the skill, to teach you how to make those choices.
Note
You might be wondering, "What should I do?" Keep making apps and don't let a lack of design experience and knowledge stop...