The oldest non-fiction book I still have from childhood is "The Stars" by H.A. Rey. The copyright page say "Fifth Printing June 1958. If I got it for Christmas that year, I would have been in sixth grade.
Before that, I remember reading a book about dinosaurs that I got from the school library in about third grade. An older girl who helped check books out of the library thought it was too advanced for a poor little third grader. To prove her point she opened the book to a random page, closed her eyes, touched her finger to the page, and said "What's this word," assuming she would have blindly pointed to a big word I couldn't read. But the word under her fingertip was "the," so I got the book. And of course, the only big words in a children's dinosaur book were the names of the dinosaurs, which were the coolest thing about dinosaurs.