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?

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WW started her carear after Batman and Supes, Bats doesn't join cause in the begining he's a loner and dosen't trust people especially people with the power to take over the planet between them, it took him ages to trust Supes and become friends

I don't know what issues, i remember reading the origin and they asked Supes to join but he said he had to much stuff to do, but if they needed any help with some thing big they could call on him