i cant understand what books it searches and why this is separately from google search at all... how do you think?...
Well Google book search uses a database of indexed books that Google has created on its own. If Google put these in the natural Google search, it could get very confusing for them when it comes to rankings. Books also tend to be less keyword targeted than websites, so a single book might be indexed under lots of keywords that the book is not really about. I think it would be bad for Google search results to include Google books; they should perhaps include a top link to the Google book results if something in the natural search seems to be closely related to an indexed book.
Books would do wonderfully in the normal SERPs, but I think Google hasn't done this out of respect to web masters who b!tch and moan all day about everything Google does. Think about it, if someone queries "favorite car" like it says in your sig, do you want to compete with The Great Gatsby? Or Chilton manuals? Google Book Search is a project where they're optically scanning books, page by page, using a non-destructive method, then using OCR software to make them searchable. They're generally starting with older books that have fallen into the public domain, stuff like Thus Spake Zarithustra.