KDE PIM is an application suite for KDE SC that helps you to organize your mail, addresses, todo's, appointments, and so on. Beyond this, it keeps your mobile phone and PDA in-sync with your desktop data.
Development of KDE PIM applications takes place mostly in KDE's git repositories. Together, the applications are about 74MB of source -- a daunting amount. These developers webpages are meant to give you a place to start, to collect documentation and explain how development within KDE PIM works.
To get started, all you really need are a few git clones. After that, you can compile and run the latest-and-greatest (and probably buggy) versions of the KDE PIM applications. When you find a bug, you can fix it, create a patch, and send it to us! That's the way KDE PIM applications are continually improving. There is much more information available to begin with, though.
Mailing lists are probably the ultimate source of development information. Follow discussions of KDE core and application developers and ask your questions. Unless you don't think about what you are saying, you will surely get an answer. The following mailing lists might be of interest for you:
The kdepim list firstname.lastname@example.org is for discussion about development of the KDE personal information management suite, KDE PIM. Read more about how to subscribe to the kde-pim mailinglist. For questions about using KDE PIM, please refer to the mailinglist kdepim-users.
The mailinglist kde-devel at email@example.com is for discussions about general KDE application development. Finally, the firstname.lastname@example.org mailing list is for Qt developers. As KDE is based on Qt, you may find this list valuable.
One of the fun ways to improve the KDE PIM codebase is to get together for a hackfest. Since most -- but by no means all -- of the KDE PIM developers are in Western Europe, getting together isn't all that hard to do. Over the years we've had seven meetings in Osnabrück, and several elsewhere.
The last meeting / hackfest was in january 2009 in Osnabrück.