KDE PIM's architecture is roughly as follows: there are a few basic libraries that provide low-level services; there are lots of libraries that are application-specific, and then there are the applications themselves. This page provides an overview of which libraries there are and what they are for.
A big part of the KDE PIM architecture is Akonadi, the data store (this applies to KDE SC 4, not KDE 3).
You may also want to take a look at the API documentation of the KDE PIM libraries and the KDE PIM applications which are generated from the source code. Remember, the documentation is intended for development use but may change without notice, be unstable, or set fire to your dog.