I can just think of one use for the phrase "What is the book?" if it will help you understand it abit... Say if my teacher was talking about a book she recommends me and I didn't hear the name of it I would answer "(and) what's the book?" or "what's the book again?" BUT like nicamamii said, it's just short for "what is the book called" basically and it's probably not "proper" english :p

这本书在那？= Where ist the book? (Where is the location /position /place of the book? Where did you put it?) You can't say 'What is the book?' Do you mean 这本书是什么问题？= What is the (/this /that) book about? (What's the topic /issue /subject of the book? What's (printed) in the book?) or do you mean 这本书怎么样？= What is the (/this /that) book like? (Did you like it? Did you enjoy it?)?

umm actually. um .. you can say "what is the book called"/// or " what is the book about" but not just what is the book unless used in a sentance while adding more details. but if you whant to know where it is then put. "where's the book?" where's = where is . good luck ! ~nicamamii~