You can use our images as main elements for the cover of a book as we understand this as a secondary element of your work since the content of the book would be the main element.
If you want to use our images inside of the book, you need to make sure to use only a few of our images and not as main elements, that are not on the full page but rather as small additions to the text in the book.
The resources used inside a book need to act as a supporting element of the book content. For example, in a book about traveling, you can use a picture of a landscape to illustrate the text.
Some usages not authorized would be coloring books, children books, or any other similar publication where all the illustrations/images are from our sites since these would be the elements that would catch the attention of the clients to buy that specific book and therefore it would be considered main elements.