You can't tell a book by "it's cover" is an old adage
It's cover begin used correctly
I believe it's should be its?
You are right.
it's means it is
its is possessive...http://www.its-not-its.info/
You are correct that "it's" should be "its" in the sentence "You can't tell a book by 'it's' cover." Here's how you can determine whether to use "its" or "it's":
1. "Its" is a possessive pronoun, indicating ownership or possession. For example, "The dog licked its paws." In this case, "its" shows that the paws belong to the dog.
2. "It's" is a contraction of "it is" or "it has." For example, "It's raining outside" or "It's been a long day." In these sentences, "it's" is used to combine "it" and "is" or "has" to form a shorter phrase.
In the sentence "You can't tell a book by 'it's' cover," the intended meaning is that you cannot judge the contents of a book by looking at its cover. So, the correct form to use is "its" because it shows possession, not a contraction.
In summary, if you are referring to something belonging to or possessed by "it," use "its." If you are trying to say "it is" or "it has," use the contraction "it's."