Probably the largest collection of texts that no longer exist is the Library of Alexandria's collection of knowledge. Destroyed largely due to religious intolerance and ignorance only a few fragments remain as well as a few titles of forever lost scrolls. These will likely never be recovered.
Les Journées de Florbelle and other works by the Marquis de Sade. Upon his death the Marquis de Sade requested that much of his work be destroyed. Unhappily some were. Again, there's little chance this will ever be recovered.
Ernest Hemingway lost, literally, an entire suitcase full of his writings. It's debated exactly what was contained in the suitcase be it original or carbon copies and from what period in his life they came from, but nevertheless, they're now lost. There is the chance that somewhere this suitcase or portions of it's contents are still floating around either misplaced in someone's attic, exchanged quietly between private collectors or buried somewhere deep in a landfill.
Louisa May Alcott's Long-Lost Novel was found unexpectedly.
Capote's lost novel was recently discovered.
And then there are the individuals such as Maurice Girodias who have produced false novels, purporting to be actual books that are only fictions themselves within a novel.
He also got into serious trouble with Simon & Schuster and author Irving Wallace over a book called "The Original Seven Minutes" by JJ Jadway which purported the be the actual book featured in Irving Wallace's title The Seven Minutes.
Bizarrely enough, this "fake" novel of The Seven Minutes (retitled 7 Erotic Minutes) is still available.