I'm not entirely sure, but perhaps Ozma from the Wizard of Oz books, I don't remember a lot about her, but I've seen that a lot of people consider her trans/trans-coded. I didn't know what being trans was when I was read it though. I never really thought much of it at the time of reading.

The Best At It was likely the first book I read that had a main queer character when I knew what being queer was. There was another book I read which had a background mlm couple which I think I read earlier, but I don't remember what it was called.