Back in February, Jonathan Lethem wrote a terrific article for Harper's Magazine The ecstasy of influence: A plagiarism From the article, speaking about researching a quotation from John Dunne:

My Internet search was initially no more successful than my library search. I had thought that summoning books from the vasty deep was a matter of a few keystrokes, but when I visited the website of the Yale library, I found that most of its books don't yet exist as computer text. As a last-ditch effort I searched the seemingly more obscure phrase “every chapter must be so translated.” The passage I wanted finally came to me, as it turns out, not as part of a scholarly library collection but simply because someone who loves Donne had posted it on his homepage. The lines I sought were from Meditation 17 in "Devotions upon Emergent Occasions", which happens to be the most famous thing Donne ever wrote, containing as it does the line “never send to know for whom the bell tolls; it tolls for thee.” My search had led me from a movie to a book to a play to a website and back to a book. Then again, those words may be as famous as they are only because Hemingway lifted them for his book title.

I cannot recommend this article highly enough.