The notation and semantics of activity diagrams have changed significantly since UML was first released. The original UML books are now out of date on the subject.
Bennett, Skelton and Lunn (2005)
Object-Oriented Technology – From Diagram to Code with Visual Paradigm for UML, Curtis H.K. Tsang, Clarence S.W. Lau and Y.K. Leung, McGraw-Hill Education (Asia), 2005