It depends whether the design of the cover can be seen as a copyright protected work; i.e. whether it is an intellectual creation with an individual character. You can usually affirm this in the case of illustrations, paintings, creative photos and designs (forms, colours, patterns) if they show the required individuality. Monochromatic and/or simply designed covers (i.e. textbooks or scientific literature) are more difficult to evaluate. Careful checks must be carried out here to determine whether the required individuality exists and if the cover nonetheless stands out from others in terms of its design and creativity. Even if the cover does not enjoy copyright protection from this time onwards, a logo or lettering of a publishing house depicted on it can potentially be protected.
In the case of book covers, it must also be kept in mind that illustrated symbols or logos or lettering can fall under trademark law.