Never judge a book by its cover.
Outward appearance is an unreliable guide to inner worth.
What They Mean
Read some pages before forming a conclusion. Presentation is packaging, and packaging is a separate industry from content.
Are They Right?
Morally, always. Practically, covers are designed to be judged, publishers employ entire departments for the purpose, and first impressions are sticky.
What They Left Out
The cover is frequently the only part of the book that was professionally reviewed.
They Also Say
Read the ruling: Clothes Make the Man vs. Never Judge a Book by Its Cover