Android

Getting Started

The Hello Android book from Pragmatic Publishers is pretty good (a substantial but easy sample project). The official Android docs are excellent, see in particular the many example projects.

Learning More

Reto Meier's Professional Android book is my favourite more advanced reference and guide. A lot of it is much clearer than the official docs (I found it particularly useful for understanding widgets, something, at the time at least, the official docs did a terrible job of).

Maximising Productivity

Projects like Android Annotations are a must. Android KickstartR bundles together a bunch of useful projects for you to get started.

Impress

Android Asset Studio is a nice way of quickly generating custom themes for your app (use with the Android KickstartR and Action Bar Sherlock for impressive Android 4.0 themes which work on older phones too).

Related Articles

Android 4.0+ and iOS 7+ Only

Why I don't think it is worth supporting Android < 4.0 or iOS < 7 on new Apps.