Q: I bought a new sefer four months ago. I now started learning it, and found it missing a whole chapter. Can I still return it?
A: You are entitled to return defective merchandise whenever you discover the defect, even years later, provided that you did not continue using the item after discovering the defect (C.M. 232:3). However, if the store has a return policy which clearly limits the time to return, you would be bound by the terms of that policy.
There is an opinion that if you had the opportunity to check the sefer when you first bought it and did not, you forego your right to claim a defect, even without a stipulated time limit. Many authorities disagree and maintain that even though you did not bother to check the sefer when you bought it, you can still return it later (Pischei Teshuva 231:1). Aruch Hashulchan rules this way, since you had no reason to assume that the sefer might be defective and had to be checked (232:5). However, since some authorities maintain that you are not entitled to return the sefer, if the seller adamantly refuses to honor the return, he cannot be forced to accept the sefer back.