In general, if an instructor requests a textbook for course reserve, then we do our best to purchase a copy. When students request a copy of their course textbooks, we turn to their instructors for guidance. For some fields - typically, those fields in which the UTRGV University Library have a historic collection strength - our bibliographers may purchase textbooks, including textbooks not used by UTRGV courses.
The UTRGV University Library purchases or loans a lot of material, but we cannot purchase or loan everything. So, recommend a purchase, explain why you're recommending it, and let us see what we can do.
If you no longer need your UTRGV course textbook and it's not heavily marked up or falling apart, then consider donating it to the UTRGV University Library!