I've just read JLA: year one and I have a few questions to which I can find no answer in the jla page here. 
 
1- Why batman and superman appear only in a very few panels? 
 
2- Why doesn't wonder woman appear at all? 
 
3- Superman joined at the end of year one, but when did WW and batman join? 
 
4- How do they become so important and why the others lose importance? 
 
5- What comics do I need to read to get all this info?