The correct sentence that follows the conventions for English usage is:
"I don't think it's that hard to read a whole book in one week if it's a good book."
This sentence correctly uses "it's," which is a contraction for "it is," in two places. The other sentences have issues with the use of "its" vs. "it's." The correct usage is "it's" for "it is" and "its" for the possessive form.