I own GGs about Weather (1987), Zoology (1958), Stars (two editions, 1956 and 1975), and Botany (1970).
All of the ones I've seen are illustrated in a very distinctive style. I consider it "mid-century" but I don't have the language to explain why. Having the entire book illustrated does give it a pleasingly cohesive feel. Though Golden Guides are still being printed today by St. Martin's Press with updated, photographic cover images, the insides appear to share the old illustration style.