Susan Howatch “The Wheel of Fortune” (1,171 pages). Fantasy.
Brandon Sanderson “Rhythm of War” (1,232 pages). Fantasy. Helped to complete Robert Jordan’s “Wheel of Time” novel cycle.
Roger Zelazny “The Great Book of Amber” (1,258 pages). Fantasy series but often seen as a single work and sold as such.
C .S. Lewis “Chronicles of Narnia” (1,288 pages) – Very famous, often considered one book and sold as such.
John Dos Passos “The U.S.A. Trilogy” (1,288 pages) – An all-time classic from 100 years ago, very popular with the Paris crowd around Hemingway, Joyce, Pound, and Gertrude Stein. Often considered a single book and sold as such.
Isaac Asimov “The Foundation” (2,680 pages) – A few books in this group, as in the title work, are very famous. Often considered a single work and sold as such.
That’s all I can think of right now. I will see if I can come up with some more.