Android design guidelines
App design is a vast topic – so vast that it could only begin to be taught in a book dedicated solely to the topic. 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 they should be, how big they should 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.
This book will hopefully leave you well-qualified to be able to implement all your chosen answers to these questions and many more besides. Unfortunately, it does not have the space, and the author probably doesn't have the skill to teach you how to make those choices.
Tip
You might be wondering, "What should I do?". Keep making apps and don't let a lack of design...