How to detect rotated object from image using OpenCV?
What features are you using to detect your books? Are you training a CNN and deploying it with OpenCV? In that case adding rotation image augmentation to your training would make it easy to detect rotated books. 
If you are using traditional computer vision techniques instead, you can try to use some rotation invariant feature extractors like SURF, however, the results will not be as good as using CNNs which are now the state of the art for this kind of problems.
I have been training OpenCV classifier for recognition of books.The requirement is recognize book from an image. I have used 1000+ images and OpenCV is able to detect books with no rotation. However, when I try to detect books with rotations it does not work properly.So I am wondering if their anyway to detect objects with rotations in images using OpenCV?