CommonsWare is here to help you through the challenges of Android app development, whether you are new to the platform or are deep into a project!
The Busy Coder's Guide to Android Development offers over 200 chapters of Android knowledge, from the very basics through highly advanced topics.
Not only does subscribing (only $20!) give you that book, but you also get additional books, including:
Android's Architecture Components, for a deep dive into this important set of Android libraries, and Android app architecture in general
Exploring Android, a series of hands-on tutorials for creating an Android app from scratch
GraphQL for Android, for accessing GraphQL-powered Web services from your Android app