In most Spanish libraries,
the stacks are not open and you
cannot browse the shelves as in an
American library. It is possible to
check out books from facultad libraries,
but loan periods are much shorter,
often 3-4 days. To check out books, you may be able to
use an electronic student ID like your UNC OneCard, but
you may need to obtain a separate
borrower's card (un carnet de préstamo)
in addition to your student ID, if needed.
Because loan periods are short and library books are highly sought after, some library users choose to do their reading, whether assigned or for research projects, in the library. For this reason, students in Spain who need access to a work for longer periods also sometimes budget to buy the books they need, when possible. Since it's not affordable or desirable to buy every book used for research, libraries tend to get more packed than what you may be used to near exam times.
To find a book for in-library use or to check out, search your university's online catalog, which will likely include books across all facultad libraries at your university. Note which library holds the item, the call number (signatura), then take the call number and student ID to a "loan" counter (préstamo) in the appropriate library. Take note of library loan policies, as penalties for late or lost books are stiff. This can sometimes include losing your library privileges--even for one overdue book!
Note that your library's catalog may not always include reference books, electronic resources, or journals. Some libraries have separate catalogs for journals and electronic resources.
Libraries in Spain use the UDC (Universal Decimal Classification) system, which is similar to the Dewey Decimal Classification you may have seen in public or high school libraries. It is called CDU, or Clasificación Decimal Universal, in Spanish. Most libraries in continental Europe use UDC. [Expandable outline of UDC]