There are two major documents about EL

A1. http://books.zkoss.org/wiki/ZUML_Reference/EL_Expressions
A2. http://books.zkoss.org/wiki/ZK%20Developer's%20Reference/UI%20Composing/ZUML/EL%20Expressions

and three(maybe more) related documents
B1. http://books.zkoss.org/wiki/ZK%20Developer's%20Reference/MVVM/Data%20Binding/EL%20Expression
B2. http://books.zkoss.org/wiki/ZK_JSP_Tags_Essentials/Working_with_ZK_JSP_Tags/EL_and_ZK_JSP_Tags
B3. http://books.zkoss.org/wiki/ZK_Developer's_Reference/MVC/Controller/Composer#Retrieve_Composer_in_EL_Expressions

I suggest:

• A2 integrate to A1
• B1, B2, B3 refer or integrate to A1