护封又称书套，是包在书籍封皮外的印刷品，广泛用于图书设计。它的高度与书相等，长度能包住书的前封、书脊与后封，在两侧的延长部分可分别向里折进前封和后封，称为勒口或折口。

关于护封最早的记录出现于19世纪20年代的英国。早期的护封像包装纸一样完全封住整本书，并用蜡或胶水封口。因此在书被打开时，护封会被撕毁并扔掉。到19世纪50年代，出现了有勒口的护封，书的内页不再被封住，遂流行于欧美地区。但由于护封上只包含有关书的简单信息且设计简陋，通常它们还是会被购书者丢弃。直到20世纪初期，出于时尚和经济的影响，出版商们弱化了对于封面的装饰设计，而通过精美的护封来吸引读者，甚至连护封的底面也出现了广告。从此，购书者逐渐开始保留护封。对此的一个例证是，这一时期原先印在书脊上的价格也转移到了勒口边上。而需要将书作为礼物的购买者，就可能会将勒口边上的价格剪去同时保留护封。

The dust jacket (sometimes book jacket, dust wrapper or dust cover) of a book is the detachable outer cover, usually made of paper and printed with text and illustrations. This outer cover has folded flaps that hold it to the front and back book covers. Often the back panel or flaps are printed with biographical information about the author, a summary of the book from the publisher (known as a blurb), and/or critical praise from celebrities or authorities in the book's subject area. In addition to its promotional role, the dust jacket protects the book covers from damage. However, since it is itself relatively fragile, and since dust jackets have practical, aesthetic, and sometimes financial value, the jacket may in turn be wrapped in another jacket, usually transparent, especially if the book is a library volume.