dear mr. ×××,
we know that you are an expert on literature. as students in english major, we want to know some details about literature. we should be very grateful if you could give a talk on "american literature" to students of the english department on saturday, ××××. if this subject does not suit you, any other similar topic would be welcome as well.
we have already had several very interesting talks from distinguished visiors from various countries this semester and we look forward eagerly to the opportunity to benefit from your experience and wisdom.
sincerely yours,
×××