I can't find a book
If the book is required for a course, there's a good chance the professor put it in Course Reserves. You can look up your course on that site, go to the library where it's being held and check out the book for anywhere from 3 to 24 hours.
If Princeton owns a copy but it's charged to a patron, send in a Recall request and it should arrive in about 1 week. Everyone is required to return recalled items, no questions asked. If a book you're using is recalled, you can immediately recall it yourself and have it back in a week.
If Princeton doesn't own a copy, search for it on BorrowDirect. Make sure to type in your barcode with no spaces (i.e. 22101565656) or else you will get an error when you try to request something. These books will usually get to you in 4-5 days, and you can keep them for 6 weeks with the option to renew for 6 more weeks (click on 'My Account' to do this).
If it's not on BorrowDirect, request the book from Interlibrary Loan (ILL). You'll need to create a username and password for this specific site—your Princeton ID and password won't work. The book will arrive in 7-10 days, and you can usually keep it for 4 weeks.
If ILL doesn't get the book to you, search on WorldCat to see what libraries worldwide hold a copy. Then try contacting those libraries directly to see how you can obtain the book, or at least some scanned sections of it.
I can't find an article
Google it. Don't mess around with PUL Quicksearch, just search for the title on Google and Google Scholar. If you're on campus, subscription sites like ProQuest or JSTOR should automatically allow you access; if not, turn on that VPN. Search results on Google Scholar will include 'Findit@PUL' links on the right that will give you more options and/or direct links to the document (e.g., '[PDF] from jstor.org').
If the full article isn't on Google, THEN try PUL Quicksearch. Remember to always click the 'Basic Search' box in addition to any relevant subjects.
If the article isn't online at all, stop being lazy and look up its journal or publication in the library catalog. It could be in Firestone's Periodicals section, it could be in the main stacks, it could be at another library. Ask for help if you need it.
If Princeton doesn't have the physical publication, request the article from ILL. They will find the article, scan it and put it online as a PDF that you can download from their site within 24-48 hours. Remember that you'll need an ILL-specific username and password.