The Hobbit by JRR Tolkien probably doesn’t need any explanation, but just in case (!!!), here’s a link to its Wikipedia page. Although I actually have to disagree with Wikipedia, which calls it a children’s classic – I think it’s just a classic. But in any case, The Hobbit is on sale today (March 25, 2021) in both the US and the UK, although the sale price is much better in the UK (£0.99) than in the US ($2.99). But it’s worth it at either price!
A word of caution – there are mulitple editions, so make sure you use the links below, and verify the price before you buy.