You will often see the Book used to mean secrets in particular. The energy of this card, however, is mainly to do with knowledge, often a deep kind of knowledge. Hence the Book is used to represent secrets in that sense: as a piece of knowledge waiting to be discovered.
The Clouds, though, because of their vague, obscuring quality and sense of confusion, are less used for the secrets themselves, but more the act of covering them up. For example, obscuring or hiding the truth, even confusing situations deliberately in order to keep something hidden. Where the Book is the knowledge itself, the Cloud could be whatever methods are being used to hide it.
Finally, the Fox again. The Fox is often used to represent both the act of deceit and a deceitful person themselves. Again, this is all part of the Fox’s energy of survival, of doing what they have to do, of being on the make, of being cunning and of deception as part of that, in order to get whatever they want.