A great book, but as you say, not really obscure.
My favourites in the category of “great-books-others-have-read-but-I-can’t-interest-RL-Friends-in” are:
“Invisible Cities” by Calvino; and
“The Man Who Was Thursday” by Chesterton.
Both are truly amazing. 
A fun book, which has two sequels, if you didn’t know about them.
Ooooh, one of my favorites. “Please accept my assurances that no personal animosity of any sort is intended towards any of you. Nonetheless, it appears, unfortunately, that I must now go forth and destroy your respective worlds. Perhaps you would like to draw straws, to determine where I might best start.”
Seconded.
Not so much an obscure book, but an obscure author; Doris Piserchia . She stopped writing back in the early 80’s but her stuff was excellent. Very imaginative. Some of her stuff includes :
Doomtime : Set in a far future Earth where the dominant life form are a pair of mountain-tall sentient trees that have extended their roots over much of the planet. They hate each other, and are constantly trying to destroy one another. They can temporarily ( or permanently ) absorb creatures into themselves, including people; this process lets them read people’s minds, and is pleasurable enough to be addictive, which is one way they control people.
Spaceling : The protaganist is an orphan girl who is a mute, a person who can see the Rings, which are drifting rings of light that are interdimensional gates to those who can see them. Anything or anyone that passes through is transformed into a new form. She, however, secretly has abilities that others don’t. She can see many more Rings than others can, her forms are different than normal people’s forms, and she can command the Rings to move as she desires. She ends up having to discover her truly odd origins, and deal with multiple conspiracies that want to help/hurt/use her.
I idolize her. Sometimes before I go out to lunch or to the theater, I think, “Now, what would Joan Crawford or Genevieve Antoine Dariaux think of this outfit? Do the shoes go with this belt? Shall I remove one accessory?”
Holy crap. I loved that book when I was a kid. I used to have a completely battered paperbook copy somewhere but I don’t think that I have even thought of that book in 20 years. Glad to know I wasn’t the only one who liked it!!
Calvin Trillon’s Remembering Denny. Unlike anything else Trillon has written, and a hauntingly good read.
Except for my wife, I have never met another fan of Penelope Fitzgerald’s The Beginning of Spring. Anyone?
Yes, she was great. I remember she wrote a really haunting and horrific short story called “We Are Idio” that would make a great radio play. One of those works you can’t forget once you read it. I didn’t like her longer works, but her short stories were just supercharged with emotion and power and originality.
Holy cow, you mean I’m not the only person who’s ever read this?
I found my copy in a Boulder used-book store in 1993; I’ve read it more times than I can count. My favorite chapter:
OK, actually I have two favorite chapters. The other reads, in its entirety:
Okay, I’m looking for a copy of Towing Jehovah tomorrow.
The book I have given away the most times is Point Last Seen by Hannah Nyala. a true account by a woman tracker of her career tracking while trying to escape an abusive partner.
After I read most books I give them away to people I think will enjoy them and tell them to pass them on. My boss borrowed this book and she then loaned it to a friend, who loaned it to a friend etc until someone lost track of it. Not knowing I didn’t expect it back she bought a replacement copy which I in turn gave back to her to loan to the person who had asked for my copy.
Sometime later I found 6 copies of it remaindered for $1 so I bought all of them and they are gone now.
Twas I who mentioned Time…Yes, you remember well. I always liked the, I believe unique, idea that human bodies can not tolerate space travel but the mind could be used to explore the final frontier. Seemed safe enough at first thought until the alien encountered started messing around with time. I should re-read that book. I haven’t read it in years!
Sorcerer’s Son by Phyllis Eisenstein. A sorcerer, Smada Rezhyk, is fearful of his neighbor the sorceress to the point of paranoia after she refuses a proposal of marriage - and it is paranoia; she barely thinks about him, much less plots against him. His particular ability is the summoning and binding of demons; these demons are basically elementals, and not especially evil. He decides to keep his neighbor occupied while he prepares certain defenses against her, and sends one of his demons, Gildrum, in human form as a “wanderer”, with orders to seduce her. He also gives Gildrum some of his own semen, whcih he ( or she; Gildrum has male and female human forms ) is to impregate her with, and then leave. This is done. The resulting son grows up with no knowledge of his father, and eventually sets out to find him.
Complicating things is the fact that Gildrum has secretly fallen in love with the sorceress, and developed feelings for the son as well; he/she can’t do much about it ( or even mention it ) , being bound to Rezhyk. An interesting part of the book is the issue of the justice of summoning and binding demons in the first place; instead of ignoring the issue like most fantasy, it becomes a major plot point.
For humerous fantasy, I recommend several series by Craig Shaw Gardner. His Ebeneezum and Wuntvor books aren’t great literature, but are quite funny. They parady almost everything in fantasy. Another funny one is his Cineverse Cycle, set in a multiverse where the laws of reality are based on movies; for example, heros never run out of bullets, but sidekicks do - so it’s very important to figure out which you are.
I also recommend the Castle Perilous series by John DeChancie whenever I get the chance. It’s humerous/serious/cosmic fantasy, centered around a castle with 144,000 doors to other worlds ( a large castle, needless to say ). His Starrigger trilogy is also good.
No one thinks of Greenland was recommended to me here and I really enjoyed it.
Another doper recommendation: Lindsey Davis’ Falco series. Well written, humourous historical mysteries set in ancient rome.
You win the clever-response-of-the-year-so-far award!
I assume they call Karl Sagan an atheist because he is not a bornagainchristian? He is a Secular Humanist. One can be quite ethical (and in many cases MORE ethical! I bet you would agree) than many professed christians.
Slightly off-topic, but if you enjoyed that you might try the Bartimaeus Trilogy, which deals with the same issue. It is touted as a kids’ book, but I would consider it to be very close to adult fantasy. I thought it was excellent - reminded me a bit of Jonathan Strange and Mr Norrell (not as complex and deep, but in atmosphere). Be sure to read all three volumes, as the first two are good, but the last one that IMHO pushes it into “rather great” territory. However, since it has 285 mostly rave reviews on amazon, I think one can hardly consider it obscure.
As for obscure, I recommend The Incredible Schlock Homes, which is hard to describe, but a really funny parody of Conan Doyle. It’s best enjoyed if you’ve read all the Sherlockiana out there, and are not averse to a pun or two. Which nobody ever has/is, as much as I try.
Two that I truly love that seem to have been missed by both casual readers and critics, but that are excellent:
Lion Country by Frederick Buechner - These days available only in the omnibus “The Book of Bebb” - the first of Buechner’s books about Leo Bebb, a sort of Pat Robertson by way of Jimmy Swaggart huckster that sells cheap ordinations in his half-phony church. The book is pure Flannery O’Connor by way of Walker Percy and early Truman Capote, though it was written in the seventies. It’s hilarious, but it bites hard.
and
Endless Love by Scott Spencer - Yes, this is the book that that embarrassingly bad 1981 Brooke Shields move was based on, but don’t let that fool you - the book is a really amazing and dense coming-of-age story set against a backdrop of the sixties’ values smashing headlong into the seventies and eighties period of the Boomers abandoning most of their core values and moving to suburbia. There’s a very heavy Holden Caulfieldism to the main character/narrator, and the actual prose reminds me so much of 70’s-era Philip Roth that I wouldn’t be surprised to learn that he ghostwrote it. Excellent.
CHOCKY, by John Wyndham
ALL THE LITTLE ANIMALS, by Walker Hamilton
THE HISTORY OF LUMINOUS MOTION, by Scott Bradfield
Oooh! Oooh! waves hand REAL high I’ve read that, and I love it to wee little pieces! It’s a little bland in places, but hey, the world’s been whitewashed. 
I should dig it out of my bookcase and read it again…
The Winter Prince by Elizabeth Wein. It’s Arthurian-based, written from Mordred’s point of view. Poor bastard. 