X-Ray feature is a one of the best which any book lover would want to have in their e-readers. Since it is named X-ray, Amazon had put it like get to explore the bones of a book. Wikipedia and Shelfari (Amazon's encyclopedia) help you get more about the book you have in your hand on Kindle.
If the Kindle book has this X-Ray feature, then the X-ray button is not grayed out. If it is grayed out, then the X-ray feature is not available for that particular Kindle book. And only English books have this X-ray feature.