Heinlein's only genuine fantasy novel is an absolute hoot from start to finish. A man answers an ad in the personal column for what is basically the perfect hunky adventurer hero. There's swords and monsters, and just about everything you wouldn't expect from the man widely regarded as the best sci-fi author ever. Worth checking out.
Pynchon struck a chord with the Sixties counterculture with this novel about a woman, Oedipa Maas, who may or may not have uncovered a worldwide conspiracy involving competing mail distribution companies. Cryptic clues involving the subversively underground Tristero postal system lead to a free association of ideas. Hard to summarise in five lines, but a fabulous read.
The Third Policeman by Flann O'Brien (1967)
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Somewhat deadly satire in true Irish style. Published posthumously, the book begins by revealing that the anonymous narrator is a robber and murderer. Thus begins the search for a black box carried by his victim which supposedly contains money, eventually leading him to enlist the aid of the three policemen stationed at the barracks of the bizarre. From there things get really weird.
Another Fine Myth by Robert Lynn Asprin (1978)
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Laced with typical Asprin-esque puns, the MythAdventures series kicked off with this relatively harmless entry and continues to this day. A powerless demon and his bumbling companion, an aspiring magician, set out on the first of many adventures. Their objective is to defeat the evil Isstvan, a maniacal magician masquerading as a kindly innkeeper. Hardly essential, but some chuckles.
Jhereg by Steven Brust (1983)
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Humorous fantasy and first in the Vlad Toltos series about an assassin who works for a mafia-like organisation. The Jhereg are one of 17 Dragaeran tribes - each of which has its own role in society. Laced with sorcery and witchcraft, the book has a fantasy feel about it that shouldn't be taken too seriously. Has its amusing moments.
On a Pale Horse by Piers Anthony (1983)
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The debut of the darkly humorous Incarnations Of Immortality series, with a book based on each of the seven supernatural 'offices'. Here a total loser is visited by Death while attempting suicide and ends up killing him. As a result, our anti-hero takes over the job and goes around on a pale horse (which can magically turn into any mode of transport) weighing souls in the balance.
Mort by Terry Pratchett (1987)
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It is difficult to split any number of Pratchett's hilarious Discworld novels, this one coming in at #65 in the BBC Big Read (with others following hot on its heels). It is the fourth Discworld novel overall and first in the 'Death' group. Death recruits the thoughtful young Mort as an apprentice. But issues of love and marriage are tough ones for a grim reaper to deal with.
Fool on the Hill by Matt Ruff (1988)
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Humorous modern urban fantasy set at Cornell University. Up-and-coming writer Stephen Titus George is in need of a good story and a better woman. With the ability to transform herself into anyone's dream babe the sorceress Calliope seems to be the answer to both. With a green canvas dragon, telepathic animals and little sprites thrown into the mix this is a lively and enjoyable romp.