Vladimir you are correct. I'm happy to see that the book is helping.
I usually recommend using "Abstract Tasks" or just Task and a work item handler implementation associated to it to implement custom logic. Even using the script task for calling your own java classes is a bad and error prone practice in my experience. Services tasks are more SOA oriented as Vladimir mention but as far as I remember they can be also used to call a method in a bean as well.. so you can try that if you don't want to write your WorkItemHandler implementation..
Cheers