YAWL User Group

Please have a look at yawl user manual

Another valuable source is the following book:

@book{ter_hofstede_modern_2010,
    edition = {1},
    title = {Modern Business Process Automation: {YAWL} and its Support Environment},
    isbn = {{364203120X}},
    shorttitle = {Modern Business Process Automation},
    publisher = {Springer, Berlin},
    author = {ter Hofstede, Arthur H. M. and van der Aalst, Wil M. P. and Adams, Michael and Russell, Nick},
    year = {2010}
}