Mark Murphy, founder of CommonsWare, is the author of The Busy Coder's Guide to Android Development, the first and largest book on Android app programming. It covers much of the Android SDK, through its 213 chapters spanning 4,292 pages.
He also wrote:
Elements of Android Jetpack, a new introductory book on Android app development, focused on second-generation techniques (Kotlin and the Jetpack libraries).
Elements of Kotlin, a guide to the Kotlin programming language, which is rapidly gaining in popularity for Android app development.
Exploring Android, a series of step-by-step tutorials for building an Android app from scratch. This book is being rewritten to demonstrate using Kotlin and the Jetpack libraries. Version 0.6 is available, with 32 tutorials spanning 472 pages.
Android's Architecture Components, a book focused on Google's new Architecture Components set of libraries with 412 pages of architectural goodness.
GraphQL and Android, a 262-page book focused on the GraphQL Web service dialect.
(yes, he writes a lot)
He offers seminars and training on Android app development topics, from "starting from scratch" introductory material to advanced areas.
Heck, he even has a blog.
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