The aphorism "you can't tell a book by its cover" originated in the times
when books were sold in plain cardboard covers, to be bound by each
purchaser according to his own taste. In those days, you couldn't tell a
book by its cover. But publishing has advanced since then: present-day
publishers work hard to make the cover something you can tell a book by.

I spend a lot of time in bookshops and I feel as if I have by now learned to
understand everything publishers mean to tell me about a book, and perhaps a
bit more. The time I haven't spent in bookshops I've spent mostly in front
of computers, and I feel as if I've learned, to some degree, to judge
technology by its cover as well. It may be just luck, but I've saved myself
from a few technologies that turned out to be real stinkers.

Paul Graham
Java's Cover - http://www.paulgraham.com/javacover.html