I don’t know if this counts, but I’m still using my calculus textbooks to keep myself sharp until I go back to school. Math textbooks don’t really become obsolete.
Still got the books from my Greek and Old English courses. I’ve referred to the Greek more recently than the OE stuff but the Greek won’t go out of date any time soon and the Old English… can’t.
Yes, often. But I am another engineer.
I’ve referred to “Modern Property Law” a few times in the 20 years since I left law school. My husband refers to his “Complete Works of Shakespeare” on occasion. But that’s it.
Constantly. I use my science and engineering texts all the time. My copy of Born and Wolf’s Principles of Optics is starting to fall apart.
I still have my humanities texts as well. I find that I have uses for them, too.
Sorta. I was an English major and am now a technical writer, so I kept (and still refer to) style guides or technical writing theory textbooks now and again.
My husband’s an electrical engineer, and he refers to some of his college textbooks frequently.
Assuming that novels for lit courses don’t count as “textbooks,” the ones I’ve refererred back to most often have been computer science, Japanese, Sanskrit, and linguistics (American English). But I’ve saved almost everything.
I’ve kept my Japanese books and a Perl For Linguists coding book with the intent to refer back to them. But beyond a desultory consultation of the former before my one trip to Japan, I haven’t really used them much.
Sometimes. I had out my college Algebra book to help my forth grader with her homework (I couldn’t remember how to do intercepts - easy once I had the reference, but I’m looking at the problem saying “I know these are really easy, but i can’t remember!”). My Accounting books come out from time to time when I do ROI work for my job (I’m not an accountant, so I don’t do that sort of thing often enough not to need a reference). I pull out some of my History books once in a while - for reference or to read for fun.
Well, I still use the textbook from the Multivariable Calculus course I took four years ago, because now I’m the TA for the same course. Not sure if that counts - “after graduation” yes, “after college” not so much.
One or two of my better math texts gets use. But in general, no.
I still (over 50 years later) have some of my math books. A few because they were classics and I cannot discard them. One, Kelley’s General Topology, I still reference regularly. What a lot of good stuff in there. I still have my English anthology, although I am unlikely to ever open it. But one that I still look at regularly was my anthropology text by Kroeber, which is still interesting to read and I do it occasionally. (He was Ursula Le Guin’s father, incidentally and he could write too.)
Absolutely I have used them. First time around, I was a Child Development major and kept the best ones (often not “textbooks” per se but texts the teacher felt were good) and sometimes referred to them, but mostly, I used them to set up a parent/teacher library at a school where I was Head Teacher for a few years. (left them there when I left).
I also hung onto some good basic English texts (Elements of Style, etc…) that everyone could stand to have on their shelves for reference, imo.
Currently, I am a Film major and have kept MOST of the books I’ve used thus far (some excellent acting or script analysis texts or plays I enjoyed working on, and also all my film course texts)
I just finished a course in the history of animation (a special focus of mine) and not only bought and kept the required text but all the other 8 books the Prof. recommended. Some great, valuable information and I refer to them all the time (and will continue to, I’m sure, as I am taking 2D and 3D animation courses next year).
Oh, and all my French texts…I also kept my son’s from his 2 yrs of it in HS.
There have only been a few that I sold back because I didn’t find them very good or think I would want to refer to them later. Guess I have been fortunate in the courses and Profs I’ve had. 
And lest you assume I am not a struggling student who could really use that extra end of term money, don’t…I am, but the buy-back is not enough to make it worthwhile to me if I like the book or think I might use it again.
I still use some of my journalism books, especially the style guides. I’ve also kept some of the better history books, but they weren’t textbooks, per se, just books about the period the course covered. I’ve got a handful of textbooks about literacy and ed psych that may be useful after graduation, but it’s too soon to know if they’ll be worth keeping.