Android became the world's #1 and most popular mobile platform for smartphones, tablets, and smart TVs long ago. Whereupon, more and more people started out studying Android application development. At this point, a good book is very helpful. The Sams Teach Yourself series of books are all good, hence here AppNee recommends Sams Teach Yourself Android Application Development in 24 Hours to Android programming beginners (a bit outdated, but worth reading).