That's not quite right. Although this is the rule generally followed in non-literary MSA, the prescriptive rules in fuSHa are a lot more complicated and will sometimes be followed even in the most basic kind of writing (you can find for example references to القبعات البيض on the internet in fuSHa sources). ناس is one of the words which is quite frequently treated as feminine singular.If the plural noun is inanimate, the adjective will be singular feminine. However, that is not the case here, so الناس لطيفة is wrong. You have to say الناس لطيفون or الناس لطفاء. However, الكتب كبيرة is correct, for the noun is inanimate. You could even say الكتب كبيرات, but that is far less common.