A variety of people have input into the ordering process. It works something like this: each department gets allocated a certain amount of money each year to buy books with, and then faculty, librarians, and yes, student suggestions too are run by the department and library liaisons for order approval. It isn't a lot of money really, considering how much academic books often cost, and then our budget has been increasingly strained over the years with no increases for inflation, and the need to pay for subscriptions to the databases like Academic Search. A typical book budget for the year is on the order of $1500, more or less depending on the department, how many majors they have etc.
The ordering is done online nowadays, through the Web Banner system. Once you log in, you will see a link for the "Library Ordering Module" at the bottom of the list. Click on that and then on "Suggest items the library should purchase." Any of our students, faculty and staff are welcome to make suggestions for ordering, just please bear in mind that budgets are limited and we can't order required textbooks for students. Given that though do make suggestions; your blogger recently approved an order suggestion for a business title from a student :-)