What literary works conventionally considered outside the canon of great works are so good that I should go out and read them immediately?

You can't go wrong with Raymond Chandler and Dashiell Hammett. Also Jim Thompson, Charles Willeford, and Carl Hiassen. Historical fiction is another unfairly slighted genre: Mary Renault, Marguerite Yourcenar, and Patrick O'Brian. I think critical opinion has gone sufficiently against Raymond Carver that it's time to consider him again. Jerzy Kosinski had some allegations made against him, but I enjoyed his small body of work a great deal. Tremor of Intent by Anthony Burgess and Possession by AS Byatt impressed both as literature and rippin' good yarns. Body and Soul by Frank Conroy and Bel Canto by Ann Patchett are great novels about music.

The Arsene Lupin novels by Maurice Leblanc. These books were most popular around the time they were written and have been largely forgotten since, outside of France. However, they are a series of quite clever detective stories featuring a gallant thief who was probably the inspiration behind more recent works such as The Saint or the Pink Panther movies. (And he also inspired the Lupin III manga and anime, which is very different from its predecessor, and the Canadian "Nighthood" animated series.)

Neil Gaiman's American Gods fits your description perfectly. This novel will be remembered--perhaps even more fondly in future generations than it is now. Neil's prose is flawless, and the story speaks on multiple levels, offering a unique picture of the origins and state of American culture (not just pop culture but the entire cultural legacy). Truly valuable literature warrants multiple readings and conveys complex ideas that require a novel's length to adequately explicate. American Gods has these qualities as well as being an exciting adventure!

I think everyone should read The Reivers by William Faulkner. It is simply the best memoir of a simple life, innocence, youth and becoming a man. I believe it is my faorite book of all time. But there are so many books to love.

Viktor Frankls Mans Search for Meaning. This is imperative!

I think Fountainhead by Ayn Rand is a good book, although it isn't considered as literature. That apart, you can also read any of Oliver Stange's Sudden westerns are a great escape. Very difficult and expensive finds, but worth.