Three books I can happily recommend:

A Princess of Mars by Edgar Rice Burroughs One of the many influences on Star Wars https://www.gutenberg.org/ebooks/62

Gladiator by Philip Wylie Supposedly an influence on the creation of Superman and modern superheroes https://www.gutenberg.org/ebooks/42914

Flatland by Edwin Abbott Abbott Philosophical and mathematical and kinda touching all at once https://www.gutenberg.org/ebooks/201

And while not sci-fi, if you’re into superheroes, Scarlet Pimpernel and Zorro are fun early secret-identify stories:

The Scarlet Pimpernel by Baroness Emmuska Orczy https://www.gutenberg.org/ebooks/60

Mark of Zorro https://www.gutenberg.org/ebooks/61620