I've noticed that some of the most demanding books also happen to be the most highly rated. 'Ulysses' by James Joyce is a prime example—its stream-of-consciousness style and dense references make it a formidable read, yet it's celebrated as a masterpiece. Similarly, 'Infinite Jest' by David Foster Wallace combines complex narrative structures with footnotes that could fill a book on their own, yet its cult following is massive.
Another titan is 'Gravity's Rainbow' by Thomas Pynchon, a labyrinth of paranoia and historical detail that demands relentless attention. Despite its difficulty, it's frequently listed among the greatest novels of the 20th century. On the fantasy side, 'The Malazan Book of the Fallen' by Steven Erikson is notorious for its lack of hand-holding and intricate world-building, yet fans adore its depth. These books prove that readers love a challenge when it’s paired with brilliance.