What are the most used textbooks for an undergrad humanities curriculum?

1. Find the website of a humanities program at a university of your choice.

2. Find the required courses for a degree in the specific area you are interested in. This is always openly published.

3. Find the course homepage for each of the courses, if available.

4. Look to see if the required list of textbooks / readings is given, which it often is.

5. Repeat as desired.

There are several excellent general recommendations below (and I've added my own bit to several of those). I'd recommend those methods as a general autodidactic / self-directed study option.

You may be able to audit courses or contact instructors directly. Both approaches are hit-or-miss, though the cost and downside risks are low.

A reference librarian, at a public library, community college (virtually all are open to the public), or a local 4-year college or university is also an excellent resource, particularly for general introductory materials.

If you have specific interest areas or goals, it might help if you clarify what those are.

Start with The Norton Anthology of Poetry [1], keep going till you're balls deep in Cunt Norton [2].

[1] https://en.wikipedia.org/wiki/The_Norton_Anthology_of_Poetry

[2] https://pen.org/cunt-norton/

If you dont want to do a minor/degree you could just go sit in in classes. Many dont take attendance. If you show up at the start of the term nobody will question it. That way you'll be able to talk to people in class and hear the prof's rants about the text as well as read it for yourself.

Open Syllabus may be of help:

https://opensyllabus.org/results-list/titles?size=50

This is a pretty broad question -- the humanities can range anywhere from literature and "critical theory" through history and philosophy to (somewhat) more "hard" subjects like sociology and anthropology. Any specific areas are you interested in?

Many of these subjects tend not to have, AFAIK, single authoritative textbooks, the way a CS course on algorithms would be likely to use Cormen et al.'s "Introduction to Algorithms" or an OS class would use Silberschatz or Tanenbaum. Those might exist for anthro and the like, but the softer humanities course tend to focus on a subject and read a bunch of selected texts around that subject. E.g. a philosophy class focused on existentialism would read Sartre's stuff, etc. The courses offered and the texts used might vary from semester to semester, even depending on the personal preferences of the instructor. These are the texts you'll tend to find on lists like this one: https://qz.com/602956/these-are-the-books-students-at-the-to....

My best general advice would be to look into the course catalog of a top-ranked university for the subjects you're interested in, and then look at the online bookstore for the texts associated with those courses. E.g. here's the course catalog and online bookstore for UC Berkeley: http://guide.berkeley.edu/courses/, https://calstudentstore.berkeley.edu/textbooks (you can search the texts by department and course).

If you're okay with a more "classical" perspective on the humanities, in the 20th century there were a few different attempts to catalog the "great books"; e.g. the Harvard Classics (https://en.wikipedia.org/wiki/Harvard_Classics and full-text available in the Internet Archive: https://archive.org/details/harvardclassics?tab=about) and https://en.wikipedia.org/wiki/Great_Books_of_the_Western_Wor....

Depending on how deep you want to go, the Oxford "Very Short Introductions" series (https://www.veryshortintroductions.com/) might be of interest. But this obviously aims more for breadth than depth.

Good luck!