I love the expression you found...I've never heard it before...and wouldn't use it now :-)
A few sayings come to mind:
- "All that glitters is not gold" (meaning that just because something looks attractive on the outside doesn't mean it is inside)
- "Beauty is only skin-deep"
- "The cowl does not make the monk" (this one is also quite old and rarely used)
To me 'the book' saying implies something is better than your first judgment, while the other sayings imply the reverse.
Hope these help anyway.