Treasures of the British Library
I am recently back from London where I visited the British Library's Sir John Ritblat Gallery. Among its treasures are the Magna Carta, the Gutenberg Bible, Shakespeare's First Folio, and the only remaining medieval manuscript of Beowulf.
There's nothing like seeing rare books in person but you can't browse through them. Now, for selected items, the British Library allows you to do that online at their virtual books page. Here you can read handwritten books by Lewis Carroll and Jane Austen, examine William Blake's, Mozart's and Leonardo's notebooks, and study the drawings in The Birds of America and the Medieval Bestiary. A "Read" option brings up the text of or commentary for the work (Carroll's handwriting is neatly legible but Jane Austen's is not). A "Listen" option reads the text or commentary to you.
I didn't learn about the treasures on the British Library website from my British Library visit. I learned by looking at the Books and Reading page at aadl.org. You can also find the Complete Works of Shakespeare, the New York Review of Books and dozens of other options there.