Dictionaries are not always on top of the changing use of words. If there are any substantial numbers of prescriptivists railing against such usages the usages are probably correct. You will find the occasional prescrip that goes out in search of typos and other simple errors and rants against them as though they were common solecisms which is why I add the "substantial numbers" caveat. Of the two choices that latishya gave I would think 'duology' would better serve the purpose. 'Diptych' would more likely bring up the image of a two part painting and then you'd have to waste time explaining that it was books you were referring to. With 'duology' you would only have to explain if it weren't books.