Not all the books on this list discuss or deal with issues of sexuality, orientation, and/or LGBTQ+ relationships; this list is to celebrate the many, many authors who did not fall in the limiting, heteronormative standards of their time (and ours) but still produced works that have become pillars of the literary canon or are just too amazing to ignore.
It should be noted that many of these writers lived and wrote in a time before our current definitions of "queer-ness" and other non-binary identities, but they are included here for their quality of their work amidst struggling with their identity and orientation during a time where understanding, resources, and sympathy were scant. I mean... they're still scant today.