As you probably guess, Forum Nokia and Symbian Developper Network are two an excellent places for documentation on Nokia Series 60 platform.

The full and updated list of doc published by Nokia can be obtained here.

Some of the must-read are - according to me:
  Getting Started with C++ Development on the Series 60 SDK by Symbian or Introduction to Series 60 Applications for C++ developers (also available in Chinese ) by Nokia are two good papers presenting in details the different steps involved in writing and compiling a simple but graphical "Hello World" application.

  Coding Idioms for the Series 60 platform which provides an introduction to the Symbian OS memory management (Cleanup stack, two-phase construction, trap harness...) and a couple of tips regarding the differences between WINS and ARM builds. Most of the stuff found in this doc is generic and can also applies to UIQ and other platforms.

  Series 60 Application Framework Handbook . This documents presents the Series 60 application framework (dialogs, views) and its base classes (CAknApplication, CAknDocument, CAknAppUI and CAknViewAppUI).

  Designing C++ Applications for Series 60 presents some example of Avkon dialogs usage (with both resource files and C++ management aspects).

  Testing a Series 60 Application that gives you an overview of techniques and macro that can be used to ensure your code doesn't fail or leak memory. As the Coding Idioms document, this doc mostly present Symbian generic techniques that apply to other GUI platforms.

  Developper's guide to Series 60 UI Category- less technical than the previous documents but provides an interesting list of UI and graphical components available on the platform.

  How to implement Direct Screen Access with a Series 60 App with associated source code