It appears that temperature affects the bookophone sound. Bookophones are generally deeply unplayable things and near-impossible to wrest a melody from, despite weeks of practice. In the summer season, books are apparently more stubborn than usual in producing tones, demanding a more vigorous action (as heard in this piece). This is perhaps because there is less discrepancy between the environmental warm temperature and the momentary heat caused by the bow friction(?). Dryness certainly deadens the tones. Anyways, it just means that in the summer, when playing the bookophone, you must really 'give it some welly'.
I can't seem to find any similar technique employed in making books 'sing', but surely over 500 years somebody must've tried something similar. Maybe a romantic poet? Incidentally, the very clever Maywa Denki laboratory has produced a musical electric book beating apparatus. (Maywa Denki received exposure in the UK some years ago with a memorable appearance of a self-playing acoustic guitar on BBC One's Adam and Joe Go Tokyo.)