I can’t believe I’m reading a book that uses the “word” thusly. I realize it’s an academic book, but “academic” does not mean you have to use pompous pseudo-words. When I came to thusly, I stopped and reread the sentence a couple times. I imagined the sentence with thus instead of thusly and saw no reason whatsoever to not simply say thus. Finally, I looked thusly up in The New Fowler’s Modern English Usage, which says:
thusly (adv.) Thusly—a word I picked up in Massachusetts and will give this single outing—if you are in that South Atlantic coaling station [etc.]—C. Freud in The Times (3 Aug. 1989). The word seemed out of order to me, since thus by itself would have sufficed. But its naturalness is more or less unquestioned in AmE (it was first recording in the OED in an American source of 1865), and a string of examples lie in the Merriam Webster files. Their usage guide (1989) declares that ‘thusly is not now merely an ignorant or comic substitute for thus: it is a distinct adverb that is used in a distinct way in standard speech and writing.’ Clement Freud’s ‘single outing’ for the word suggests that it has not been washed ashore in the UK yet (p. 782).
My suspicions were confirmed: don’t use thusly! If I owned a copy of Grammar Snobs are Great Big Meanies, I’d look it up in that book, too, just for entertainment.