A few of these contain canonical m/m UST rather than a m/m relationship. Also, I'm sure I've missed a bunch--additions very appreciated!

The Administration - Manna Francis
Alexander Trilogy - Mary Renault
Aristotle and Dante Discover the Secrets of the Universe - Benjamin Alire Sáenz
Astreiant Series - Melissa Scott & Lisa A. Barnett
Band Sinister - K. J. Charles
The Binding - Bridget Collins
Black and Blue Series - Lily Morton
The Bone Key - Sarah Monette
The Charioteer - Mary Renault
A Charm of Magpies Series - K. J. Charles
Dark Matter - Michelle Paver
David Blaize - E. F. Benson
England Series - K. J. Charles
False Colors - Alex Beecroft
For Real - Alexis Hall
The Gentle Art of Fortune Hunting - K.J. Charles
Hexworld - Jordan L. Hawk
Honeytrap - Aster Glenn Gray
The House in the Cerulean Sea - T. J. Klune
In Other Lands | The Turn of the Story - Sarah Rees Brennan
Integrate - Thea Hayworth
The Last of the Wine - Mary Renault
Lilywhite Boys Series - K. J. Charles
Lord John Series - Diana Gabaldon
Lynes and Mathey Series - Amy Griswold & Melissa Scott
Machineries of Empire Series - Yoon Ha Lee
The Monster of Elendhaven - Jennifer Giesbrecht
Nightrunner Series - Lynn Flewelling
Page & Sommers - Cat Sebastian
Peter Darling - Austin Chant
The Price of Meat - K. J. Charles
Pricks and Pragmatism - J. L. Merrow
Rath & Rune - Jordan L. Hawk
Regeneration - Pat Barker
Seducing the Sedgwicks Series - Cat Sebastian
Simon Feximal Series - K. J. Charles
Sins of the Cities Series - K. J. Charles
Smoke Series - Tanya Huff
Society of Gentlemen - K. J. Charles
Something Human - A.J. Demas
Sun Moon Dust - Ursula Vernon
The Tarot Sequence - K.D. Edwards
The Turner Series - Cat Sebastian
The Watchmaker of Filigree Street Series - Natasha Pulley
Whyborne and Griffin - Jordan L. Hawk
The Will Darling Adventures - K.J. Charles
Winter's Orbit - Everina Maxwell