SOME fearless outside referee had to barge in and try to adjudicate the culture wars, so let us rejoice that it’s Zadie Smith. She brings almost everything you want to the task: humor, brains, objectivity, equanimity, empathy, a pitch-perfect ear for smugness and cant, and then still more humor. Born in 1975 – safely past the 1960’s, the birth of our blues – she’s not much burdened by heavy dogmatic baggage of her own. Being from England, she is one wry remove from the ground zero of these battles, America. She can’t reconcile the warring camps – no one can – but “On Beauty” is that rare comic novel about the divisive cultural politics of the new century likely to amuse readers on the right as much as those on the left. (Not that they’ll necessarily be laughing in the same places.) Yet Smith is up to more as well: she wants to rise above the fray even as she wallows in it, to hit a high note of idealism rather than sink into the general despair. How radical can you be? Blame it on her youth.