The Morgan Library & Museum, New York, NY

The first time I ever heard about this library was by absolute accident: fellow subscribers of a mailing list were discussing what to do on a visit to NYC and someone recommended The Morgan saying that there are manuscripts by Jane Austen and Charlotte Bronte and Charles Dickens there, among others. So of course I had to know more and the more I learned the more I wanted to go there immediately and look at all the treasures with my own eyes.
The library was founded in 1906 to house the private library of J.P. Morgan and was made a public institution by his son in 1924. It is now a museum and research library and houses priceless manuscripts, diaries, correspondence and books from all over the world.
Here's what's there, from the library's website:

A centerpiece of his collection was—and still is—the sole surviving manuscript of John Milton's Paradise Lost, transcribed and corrected under the direction of the blind poet. Other collection highlights are Charles Dickens's manuscript of A Christmas Carol, Henry David Thoreau's journals, Thomas Jefferson's letters to his daughter Martha, and manuscripts and letters of Jane Austen, Charlotte Brontë, Lord Byron, Wilkie Collins, Albert Einstein, John Keats, Abraham Lincoln, and John Steinbeck.
[...]
The Morgan's collection of literary and historical manuscripts has been enriched by many gifts and acquisitions, and twentieth-century holdings have increased significantly. The collection, particularly strong in artists' letters, was greatly enhanced by the Pierre Matisse Gallery Archives, the gift of the Pierre Matisse Foundation in 1997. These archives include more than fifteen hundred letters as well as records of the gallery installations of Balthus, Chagall, Dubuffet, Giacometti, Miró, and other twentieth-century artists. The Carter Burden Collection of American Literature includes important manuscripts and correspondence of John Cheever, Ezra Pound, and Tennessee Williams. The 1999 acquisition of The Paris Review Archive added correspondence, interviews, typescripts, and revised proofs of several hundred post-World War II writers, including Donald Hall, Ernest Hemingway, Jack Kerouac, Norman Mailer, Toni Morrison, George Plimpton, Philip Roth, and Anne Sexton.
[...]
The [Printed Books and Bindings] collection's strong base derives from the major acquisitions of Pierpont Morgan, who sought to establish in the United States a library worthy of the great European collections. It is rich in special and unique copies, first editions of classical authors, and works of notable printers, such as Jenson and Caxton. Among the highlights are three Gutenberg Bibles, a strong collection of works by Lord Byron, Charles Dickens, Edgar Allan Poe, John Ruskin, Mark Twain, Herman Melville, and William Morris, and classic early children's books. The Carter Burden Collection of American Literature, a major 1998 gift, strengthens the Morgan's twentieth-century holdings with authors such as Ted Hughes, Sylvia Plath, Vladimir Nabokov, Gertrude Stein, and Tennessee Williams.

I could go on and on but I really want to go back to the website and explore some more so that when I finally get there some day I'd know my way around and won't completely fangirl over Jane Austen's manuscripts. Hopefully.