It says that you can't remove or replace the cover of a book and then sell it.
This bit of "legalese" is explained by a feature of the publishing industry.
A book publisher needs to get his books into bookshops. To encourage bookshops to stock their books, the publisher sells them on a "sale or return" basis.
The publisher sends his books to the publisher. The bookshop pays for the books, but if any books remain unsold, the bookshop can return the books to the publisher for a refund.
Returned books will be destroyed. It is standard practice for the bookshop to save on postage by not returning the whole book, but only the cover. The bookshop will then destroy the remainder of the book.
The requirement not to sell under a different cover is to prevent a corrupt bookshop from removing the cover, returning the cover to the publisher for the refund, putting a new cover on the book and selling it.
This is directed at bookshops, not the final buyer of the book.