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.
Course catalogues are something of a dying phenomenon, but may still exist in either printed, PDF, or nominal online form, and can offer detailed information on numerous departments, majors/degrees, and programmes.
Note that required courses are often mentioned only by number, such that you'll have to cross-reference these to find the actual courses involved.
(This practice made some sense in the days of print catalogues and increased costs of additional detail, less so in an age of online presentation.)
If the course itself doesn't list reading materials, both on- and off-campus bookstores will typically list the reading lists for at least the present session's (quarter or semester) courses.
Instructors also frequently independently list syllabi on their own faculty webpages, though this also seems to be a slowly dying practice, and more commonly practiced in technical departments (with computer science probably the most representative), least often in humanities, business, and professional degrees (law possibly the least likely to do so, the role of the legal profession as an informational gatekeeper goes back millennia).
The Internet Archive can be useful in finding prior years' syllabi and course descriptions, especially where these have disappeared behind registration walls / proprietary education information systems (this seems to have occurred largely ~2010--2019 from my own explorations). Actual content change is usually remarkably slow even in relatively rapidly-advancing fields, especially in lower-division (that is, elementary basics) courses. A notable exception from my own uni years was the geography course on the Soviet Union for the Winter 1992 section. Whilst not enrolled in the course, I remember seeing the extensive supplemental reading in the on-campus bookstore at the time.
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.
[1] https://en.wikipedia.org/wiki/The_Norton_Anthology_of_Poetry
I was going to suggest looking at course syllabi myself. These are less available online than they once were as uni education moves to closed, registration-based platforms, but can still often be found.
"The Great Courses" series is a bit mixed, but also tends to cover the basics in the more popular topics. They're often available at libraries and the references / readings can be cribbed.
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!