EDK is not easy. Using EDK assumes knowledge of MAPI. Some examples of using Extended MAPI in DELPHI 5 you can find HERE. If you know how to use MAPI then you will most likely benefit from using EDK when trying to write programs for Microsoft Exchange Server. It makes Exchange developer's life a little bit easier, that's all.
EDK is a large collection of functions, interfaces and sample code. Most of the functions have the Hr prefix, such as HrEnumSites. Interfaces are usually prefixed by IExchange, for example IExchangeFolderACLs.
MSDN library covers EDK in the following section:
SDK
Documentation/Platform SDK/Database and Messaging Services/Microsoft Exchange
Server.
As you can see, it is considered separate from MAPI. Messaging API (MAPI) section follows Microsoft Exchange Server section. This is good for the reason that MAPI is an overwhelming collection of things. If someone wanted to get familiar with MAPI he or she should be prepared to spend many months just learning about it. Even after that it is easy to get lost doing simple things. The collection of MAPI interfaces is large, its coverage is unprecedented. MAPI is generic and universal and comes with a price of nobody being able to understand it completely.
EDK should help you to handle MAPI programming in a more organized way. It only applies to programming for Microsoft Exchange Server. But if this the environment that you wish to have programmatic control over, you should get familiar with EDK.
The Microsoft Exchange Server section has three parts: