1950-present
I've intentionally constructed this from an anglocentric perspective, prioritizing obscure works in English and including works well-known in literary circles in their original language/country.
Underread does not mean obscure, so some of these are decently popular in the anglophonic world (by which I mean they've been read by many of those with an interest in the unique) yet are surpassingly well-written.
All of these I've read and enjoyed or my girlfriend has. If you have a question about any, ask me in the comments and I'll reply or relay the question and answer.
I'm using avant-garde and experimental in the broadest possible sense, and some of these are hardly formally unique.

Love it. The only one I’ve read is the excellent Motorman. I’ve read other stuff by Matthews and B.S. Johnson; the rest of this stuff is terra incognita for me.