The Microsoft Dynamics CRM 4.0 SDK is an essential part of your toolit if you are developing against CRM, the latest release is version 4.0.7 and can be found on the Microsoft download site.
The SDK has been changing fairly frequently so it is a good idea to check back regularly to see if an updated version has been placed.
About alistairl
After starting my career as an accountant, I veered off and commenced a career in software development. Over the years I have concentrated on Microsoft technologies, and participated in the associated certification programme, gaining MCSD, MCDBA and MCPS in relevant subjects. I have been part of and led teams in some of the major Internet developments using Microsoft Technologies in Scotland, and now combine development and business analysis work for Charteris. I have broad experience but recent focus around SharePoint and Dynamics CRM 4.0 integration.