I found a shadow image in the bundle of iBooks app in iOS 12. I think Apple use this image to create book's shadow, but I don't know how they do it.
shadow image using in iBooks app
My solution is scaling the shadow image to the appropriate size and put it behind the book cover image. But for cover images of different sizes, the shadow radius are different, the image below shows what it looks like, the effect is different from iBooks.
This problem has troubled me for a long time, I need help. Thanks a lot!