Never judge a book by its cover - никогда не суди книгу по обложке.
Reasons against
1) nowadays book covers can say a lot about book topic or a plot. It has pictures that can give you an idea about it.
2) nowadays it's popular to put book reviews on a book cover. So you can see opinions of other people about the book.
3) If the book has remarkable cover it may mean the book is interesting itself.
4) on a book you can find a name of an author. If the author is famous or you already read some of his books you can will his new book be good or bad.