I got this sentence from Magic tree house series.
"What's the book say about her?"
Although I can understand the meaning, I can't exactly know what's means.
Is it 'what is' or 'what does'?
There are of course many non-native teachers in the world. However, I would expect them to be able to tell their student which one of "What is the book say about her?" or "What does the book say about her?" is the correct version.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
Usually 's stands for is or has. In fact, I would have given only those possibilities. But then you come across this construction and it immediately becomes clear that what's is what does.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.