Archive.org has been an absolute godsend for historians and others who use rare books. Case in point: when I was doing PhD research in Lisbon three years ago, I had to search several rare book shops and ended up paying 80 euros for a very rare 19th century Portuguese book I needed for my research. Here it is on archive.org in multiple editions, all text-searchable: https://archive.org/search.php?query=duarte%20ribeiro%20de%2...