One of the primary traits of a good developer is a strong desire for excellence. If a developer is willing to settle for a mediocre and boring design, then that is some (perhaps small) evidence that the book may contain mediocre and boring content.
Anecdotally: At the TechWeek Chicago job fair, there was a very high correlation between booths with good logos and graphic design and interesting people behind the booth. All of my good conversations were with people at the well designed booths.
EDIT: This is especially true since we are expected to pay for the book before we get access to it. This means that we cannot judge its quality in any holistic way, but must do our best to pre-judge its quality through various social signals.