The most commonly assigned book in US colleges is…
…Frankenstein! According to Degree Query, which used information from the Open Syllabus Project to compile lists of the most commonly assigned books across disciplines, as well as compare what students at public versus Ivy League colleges are reading.
At public schools, William Strunk Jr.’s The Elements of Style is the most frequently assigned book, while in the Ivy League, Thomas Hobbes’ Leviathan (which, as I recall, is about… society? And a sea monster?) takes the title. The Elements of Style, The Communist Manifesto, and The Tragedy of the Commons crack the top ten in both cases.
Sadly, though not surprisingly, the survey also finds that only 20% of the most assigned authors were women, including mother-daughter dream team, the Marys Wollstonecraft and Shelley, as well as Kate Chopin, Judith Butler, Charlotte Perkins Gilman, and Diana Hacker.
You can take a look at the whole survey (and perhaps create your own D.I.Y. syllabus, because higher education in this country is largely a scam?) right here.