The official sample code and tutorials can be found here: http://developer.android.com/resources/index.html
For reading code I can recommend these repositories:
More sample applications for the Android platform: http://code.google.com/p/apps-for-android/
Open Source examples from the books http://commonsware.com/AndTutorials/ and http://commonsware.com/AdvAndroid/ are available at http://github.com/commonsguy/cw-andtutorials and http://github.com/commonsguy/cw-advandroid/ . They helped me a lot by simply reading the code and searching for classes I needed.
You can take a look at the source code of the common Android applications like Contacts, Email, Calendar, etc.
Look at projects under platform/packages/apps here:
https://android.googlesource.com/platform/packages/apps/