I've loved and read SF since I was about 10. And I too read and really liked
Steel Guardian; I'm so glad you mentioned it,
@Rose65; I kept forgetting to read the 2nd in the series; going to go online and look for it right now, lol.
Some of my favorite SF authors: Jack McDevitt, James Tiptree, Sheri Tepper, Arthur C. Clarke (especially his book
Childhood's End--the miniseries was okay but loved the book, I wish so much I could join the Overmind and leave physicality behind!), Stephen Graham Jones (horror rather than SF but so good), Kiersten White, Alma Katsu, Marilyn Messik (hers are paranormal but handled in a matter-of-fact way that reads like SF, very good), and many others.