With the arrival of the 5th grade in the LLL during the construction of the new Lower School building, access to the LLL will be limited. Please use the Google form to request a book.
Both Montgomery County Library and Prince Georges County Library are excellent resouces. In addition to print books, there are many e-books. You can place holds and even request books via Interlibrary Loan (ILL) if the library doesn't have what you need. If you are placing a hold expect it to take a few days for something that is in, but located at another branch. For ILL it may take a week or more to come in.
DCPL has a really strong collection and will deliver a book you need to your local branch when you place a hold. DCPL does participate in Interlibrary Loan (ILL) but there may be a fee associated with it, so it is best to check with the library before placing a request.
The Senior Research Seminar travels to AU to conduct research in the AU Library. You can look in their catalog now to get an idea of what you might be interested in. It is also possible to walk in and look at resources on site.