WeLib
welib.st
Massive online library claiming tens of millions of books and academic papers with direct search for books, articles, magazines, comics, and downloadable formats.
Why it is useful
Book search and paper search live in one interface, which makes it unusually effective for chasing hard-to-find references across academic and general reading. It functions less like a polished scholarly database and more like a deep shadow index of published knowledge. For stubborn source hunting, that breadth beats many cleaner but narrower tools.