My college library has many class texts on the reserve shelf. This allows students who might not be able to afford a personal copy of a text to borrow the text for a few hours or days (depending on the item's lending policy).

Even when I was an undergrad, the campus library had textbooks on reserve. I mostly used this to access the questions from the current edition of the textbook, so I could use the cheaper previous edition. I promise you, Mechanical Stress Analysis doesn't change much, year to year.