Beautiful cover with picture of an opening through a forest. The colours were browns and golds and green. It was a story about a boy and girl traveling into an enchanted place and the things they experienced there, an adventure. I read this book over 50 years ago.
This sounds like an illustration from “The Magician’s Nephew” by CS Lewis (one of the books in the “Chronicles of Narnia” series. The earlier hardback versions had some beautiful illustrations by Pauline Baynes of Digory, a boy, and Polly, a girl, in a place called “The Wood Between The Worlds”. You can Google “Magician’s Nephew illustrations” or Wood Between the Worlds illustrations” and see them online–they do have lots of greens and browns and golds. I am not sure if the pictures of the wood were on the cover of those early versions or just inside the book. The wood in the story is the gateway between many worlds, one of which is Narnia, another is Charn, where Digory and Polly have an adventure and meet the evil witch Jadis, who follows them first back to London and then to Narnia, where they have numerous adventures. It is a prequel to “The Lion, the Witch and the Wardrobe”.