Here you can find some useful references/links about the android development, that I find valuable.
Android Wireless Application Development
"Android Wireless Application Development combines all the reliable information, sample code, and best practices you need to build, distribute, and market successful Android mobile applications."
The official Android Developers APIs website.
A tutorial on writing a complete notepad application.
A website full of useful code snippets for the android platform.
Understanding User Interface in Android
A guide for making user interfaces in an Android application (layouts, views, widgets, etc.).
A Visual Guide to Android GUI Widgets
A list of some of the available UI widgets. Very helpful for quick reference.
Recognize/React on incoming SMS
A tutorial about how to listen to incoming messages events.
Signing Your Applications
Official information about signing Android applications prior to publishing them for mobile device users (e.g. on the Android Market).
Icon Design Guidelines
Some guidelines for creating nice icons for your application.