The richest source of information about xtUML is found on xtuml.org.
Specifically, there is a ‘Learn’ menu item with all kinds of resources.
For a high-level overview, I recommend MDA Distilled.
For deeper Executable UML, I recommend Mellor/Balcer and then Rastrick.
An interactive online training course is available that focuses on xtUML and the BridgePoint tool (requires a free login to the site). The course includes dozens of videos followed by quizzes. You can progress through the material at your own pace or pick and choose which topics to learn. All of the training materials are open source and in the Public Domain.
You can download BridgePoint, which is Open Source Software (Apache 2.0).
The Nightly Build is the latest and greatest.
If you want to join the xtUML Community as a developer (of BridgePoint and xtUML models), there is a ‘Developer Getting Started’ guide that walks you step by step through building the tool.
Professionally delivered training and consulting is available from One Fact and others.