Stephen King's The Dark Tower series of novels are probably his most underrated, and that can mostly be attributed to the fact that it took him 22 years to finish the series. It's a sprawling epic story, starring The Gunslinger, Roland, and his companions as they are inexorably drawn towards the titular Dark Tower. Like The Lord of the Rings, it's a travel story, with all of the action happening during the journey itself.
If you haven't read the series, I can't recommend it highly enough. It's got gunslinging, swords and sorcery, time travel, interdimensional doorways, artificially intelligent monorails, and so much more. It also ties most of King's major novels together in bizarre ways, without getting boring. Either pick up the first book in the series and check it out, or listen to the audiobook during your commute. You won't be sorry.
Just do it before J.J. Abrams and his Lost crew begin making the movies. Wait, what's that? Abrams? Lost? The Dark Tower? Read on to see how all of these pieces will soon fit together to produce what some claim is destined to become the next Lord of the Rings.