Hi,
in an attempt to improve my French skills I would like to read a couple of books in French, preferrably 20th century classics and preferrably non-translated works. What would your best suggestions?
For reference, I like authors Jorge Luis Borges, Jean-Paul Sartre, Pär Lagerqvist, Haruki Murakami and Aleksandar Hemon.
Well, Sartre was French, so you could read him. 
Camus would seem to be an obvious choice given the authors listed.
I don’t read or speak French, but if you like Borges you might want to try reading Jules Verne in the original language.
L’Etranger is not too tough in the original French.
Jean Moulin’s Premier Combat hasn’t been translated, is regarded as a non-fiction classic in France and is written in a plain, unadorned style that’s relatively easy to read. It’s his contemporary account of being the prefect in Chartres during the invasion in 1940, prior to him joining the resistance.
What’s your skill level? It may sound odd but you should consider breaking yourrself in with bandes desinees like Les Schtroumpfs (The Smurfs), Asterix, and Tintin. IIRC there are some quite subtle - and not-so-subtle - jokes. Reading newspapers is also good.
Hmm… paging clairobscur.
Sartre, “Les Jeux Sont Faits” would be good. Maybe Gide’s “La Symphonie Pastorale.”
The Little Prince.
I quite liked Camus’s dramas “Caligula” and “Le malentendu”–those are good places to start.
Les Schtroumps and Asterix are written for a younger audience. Tintin is a timeless classic, and the later stories might appeal to an older reader - let’s say from “Le Secret de la Licorne” on.
But after seeing the preferred reading list of the OP, I think that more “adult” comic strips would fit the bill better. By “adult” I don’t mean X-rated - many comic books (what you would call “graphic novels” in the USA) are written for older teenagers and/or young adults. I don’t follow the latest much anymore since I’ve lived in the USA, but a series like Le Combat Ordinaire might appeal to the OP (a photojournalist tries to decide what to do with his life.)
Not a bad suggestion really, but being European and grown up with comic books, I’ve already read them all (including Gaston and Spirou).
My skill level… Pas trop avancé. But I have a good understanding for languages overall and have spoken to french people a few times.
L’Etranger sounds like a good choice.
More votes for Le Petit Prince (for starters), and then whatever Camus tickles your fancy.
Next, Proust. 
Here I was going to come and suggest you pick up “Le Petit Nicholas” and have fun
Then, go read some Daniel Pennac - La Petite Marchande de Prose, Au Bonheur des Ogres, Monsieur Malaussene, and so on. You’ll have a blast with those…
Pennac is fun. His essays are also a blast.
Eric Emmanuel Schmitt too – anything of his…
Elly (francophone, native speaker)
Well, the Smurfs are written for a younger audience (although they do contain a few jokes that I didn’t get until I got older), but I’d personally say that Asterix is actually more “adult” than Tintin. Asterix albums (especially the Goscinny-era ones) are accessible to children, but contain a large amount of social satire and cultural references that children just won’t get.
However, all of these are still a rather easy read, so I agree that they’re not quite what the OP is looking for.
Am I the only one who remembers that Saint-Exupery wrote books OTHER than just Le Petit Prince?
I found his other novels adult enough to be interesting but written with language straight-forward enough for my not-spectacular French. Titles like Vol de Nuit, Terrre des Hommes, and Courier Sud might be suitable for your needs.
A couple more play suggestions:
Rhinoceros, by Eugene Ionesco
Antigone, by Jean Anouilh
were both interesting, and not to difficult to parse.
I’d discommend L’Etranger, but that’s because I hate the main character.
Papillon, by Henri Charrière, can’t be called a classic, but it’s a ripping good read, and really not too difficult despite its length.
Chère voisine, by Christine Brouillet (Quebecker now living in Paris), isn’t a classic either, but a very well-structured thriller. Not tough to read at all and better-written than the average thriller.
If you’d like something that had the literary (and legal) world hopping, try Les Particules Élémentaires or Plateforme by Michel Houellebecq.
And of course, no discussion of French writing would be complete without mention of La vie sexuelle de Catherine M., French art critic Catherine Millet’s very frank discussion of her sex life.
God, I’m low-brow when it comes to French writing, but I assume you don’t need us to mention Molière, Hugo, Balzac, etc.
Well, I don’t read French, but I do dearly love the English versions of these books by Marcel Pagnol:
Jean de Florette
La Gloire de mon Pere
La Chateau de ma Mere
His stories are a bit on on the straightforward, deceptively simple side, so I’ll assume that they’d be good starter reading in French.
Of course, you can’t read Jean de Florette without reading the second part, Manon des Sources. A Greek tragedy set in Provence.