Adobe Live docs

Vary useful flex 2 live docs by Adobe on the path below
http://livedocs.adobe.com/flex/2/

You will find here docs about :-
Getting Started with Flex 2 – It contains an overview of the Flex Product family and series of tutorials
Flex 2 Developer’s Guide – It covers all the components their default values & how to use them with source. It also covers using events, creating custom events, using style & themes, skinning the components, shared objects, history management and many more things. This guide is useful when you are new to flex and creating your application in flex.
Building and Deploying Flex 2 Applications – This covers process of building & deploying your application right from development directory structure, Flex 2 SDK and Flex Data Services Configuration, Using the Command-Line Debugger and using Runtime Shared Libraries.
Creating and Extending Flex 2 Components – It covers creating simple as well as advance components using MXML and using ActionScript, creating custom Formatters & validators
Programming ActionScript 3.0 – It covers introduction to AS 3, display programming, player API overview, event handling, networking & communication and using external API.
Using Flex Builder 2 – It has details about using flex editor details & using flex.
Flex 2 Language Reference – It covers all the packages, Interfaces, classes of Flex 2.0. It also provides all the details of the class right from public methods, public properties, events and some details about each.
Flex Data Services 2 Java Reference – It covers details about flex data, management, and messaging packages. Classes nad packages inside these them with details of each class and their members  

You can download the PDFs of below docs to use offline

Getting Started with Flex 2
flex2_gettingstarted.pdf
Flex 2 Developer’s Guide
flex2_devguide.pdf
Building and Deploying Flex 2 Applications
flex2_buildanddeploy.pdf
Creating and Extending Flex 2 Components
flex2_createextendcomponents.pdf
Programming ActionScript 3.0
prog_actionscript30.pdf