Non-mainstream is in the eye of the beholder, but let me try offering some fantasy recs that don't fall into the Tolkienian epic fantasy mould...
"Jonathan Strange and Mr Norrell" by Susannah Clarke "The Light Ages" by Iain MacLeod "The Glass Books of the Dream Eaters" by Gordon Dahlquist "City of Saints and Madmen" by Jeff VanderMeer "Perdido Street Station" by China Mieville "Gormenghast" (a trilogy) by Mervyn Peake "Verdigris Deep" by Frances Hardinge (a YA book, but who's counting?)
Is that the sort of thing you're looking for, or is that still too mainstream?
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"Jonathan Strange and Mr Norrell" by Susannah Clarke
"The Light Ages" by Iain MacLeod
"The Glass Books of the Dream Eaters" by Gordon Dahlquist
"City of Saints and Madmen" by Jeff VanderMeer
"Perdido Street Station" by China Mieville
"Gormenghast" (a trilogy) by Mervyn Peake
"Verdigris Deep" by Frances Hardinge (a YA book, but who's counting?)
Is that the sort of thing you're looking for, or is that still too mainstream?