I suppose he's best known for writing the Surrealist Manifesto in 1924. (Click on that link if you like, but be warned that nearly every version of it that I can find on the web seems to be slightly different from every other version...) Surrealism is often defined by the random juxtapositions and dream-like states often seen in its art and literature, but there was a very spiritual side to it, too. They were thinking of calling it "supernaturalism," after all! Breton almost seemed to worship the surprises that can come from the unconscious...
Let us not mince words: the marvelous is always beautiful, anything marvelous is beautiful, in fact only the marvelous is beautiful.
The mind which plunges into Surrealism relives with glowing excitement the best part of its childhood.
But it was more than just letting the unconscious have its playful free reign (like in "automatic writing"). The French prefix sur- means above, and I think Breton was trying to travel through these strange, dreamy realms to reach something higher... "to express -- verbally, by means of the written word, or in any other manner -- the actual functioning of thought."
This reminds me of a more recent book, The Passions by Robert Solomon. That somewhat dry, scholarly book described some interesting new theories about the role emotions play in our lives. Solomon coined the phrase SURREALITY to describe our own subjective view of the world. The exterior, consensus reality is there, but overlaid on top of it is a thick layer of our own feelings and notions about the things and people that we see. Everything we interact with is "tagged" with meaning -- in fact, one could even say that meaning itself exists mainly in surreality. Was this the same "higher reality" toward which Breton was pointing a finger?
I called Breton an incurable romantic. Throughout his life, dude was married 3 times. In the 1920s, he fell hard for the progenitor of all manic pixie dream girls (heavy on the manic, unfortunately), and waxed poetic about her in his semi-autobiographical novel Nadja:
"I have seen her fern-colored eyes open mornings on a world where the beating of hope’s great wings is scarcely distinct from the other sounds which are those of terror and, upon such a world, I had yet seen eyes do nothing but close."
I chuckle a bit now at the over-earnestness of it all, but at age 20 or so, when I discovered Nadja, I was all about the over-earnestness. :-) Breton grounds out his surrealist weirdness with a series of photographs of the places in and around Paris where the action took place. I took the book with me when I first saw Paris in my early 30s, but I didn't follow through on my plan to pilgrimage... there were enough wonders to see without having to retrace anyone's steps!
I've read that in the last few decades of his life, Breton got more interested in various occult topics -- Tarot cards, Western Hermetic Magic, and so on. He called it "a multi-layered game ... for stretching and exercising the mind." (Much like how I think of the Glass Bead Game.) I hope to someday research that part of his life more deeply.