Great topic Sam!
One of my all time favourite books is The Long Way to a Small Angry Planet by Becky Chambers. It's not super heavy on the queer aspects of the book, but it does have (SPOILERS) two female characters who enter into a relationship, and I particularly enjoy that there's no huge deal made about the fact that the two characters are of the same gender. The bigger deal is about a much bigger difference that I won't spoil. If you loved Firefly, you'll love that this book has the same warm and fuzzy feeling. And there is no horrible tragedy that befalls the couple (so far as I can remember). I've used this book for a queer book club, so I can tell you that an entire (small) book club of people also liked this book.
The sequel is equally awesome, although it follows some characters that were only side characters in the first book. It's called A Closed and Common Orbit. I hesitate to say that this book is LGBT exactly, but I mention it because there is an alien character whose (SPOILERS) species changes sex periodically. This aligns, for this character, with changes in physical appearance, presentation, role within the species, and most importantly perception by others. Pronouns also change. It is still an interesting view, in my opinion, of perceptions of sex and how that influences perceptions of gender, and how that can impact people. Note that this is not the focus of the book.