The bulk of our developer documentation and source code activity lives on GitHub in and around our many GitHub repos. That's where you should go if you're interested in extending xTuple with scripted packages, modifying core source code, and interfacing with third party applications.
We are in the process of updating our developer documentation and migrating it all to GitHub. Links to some of the remaining documents which have not yet been migrated can be found below. To get started quickly with questions related to the C++ rich client application, please see the Qt client wiki on GitHub. xTuple database server information can be found in the xTuple admin utility wiki on GitHub. And for the web client, check out the web client wiki on GitHub.