Your harmony book sets a high bar for obscurity! I'm sure many of us with an interest in harmony would like to know more - please tell us something about it.
A book I am very fond of that I don't think is widely known (though it's not in the same league as your suggestion) is "Resisting the Virtual Life", a 1995 collection of essays on the theme of cyber-wariness published by Lawrence Ferlinghetti's City Lights imprint - more often associated with poetry.
Also very obscure for a long time, though easily bought now: Mervyn Peake's self-illustrated children's book "Captain Slaughterboard Drops Anchor". From the author of Gormenghast, but frankly much better. Highly recommended.
A book I am very fond of that I don't think is widely known (though it's not in the same league as your suggestion) is "Resisting the Virtual Life", a 1995 collection of essays on the theme of cyber-wariness published by Lawrence Ferlinghetti's City Lights imprint - more often associated with poetry.
Also very obscure for a long time, though easily bought now: Mervyn Peake's self-illustrated children's book "Captain Slaughterboard Drops Anchor". From the author of Gormenghast, but frankly much better. Highly recommended.