The Neverending Story by Michael Ende
If you remember the movie of the same name from the 1980s, then you already know the first half of this story. It’s about a boy and a magical book and the realm of Fantastica (called “Fantasia” in the movie). That part of the story is quick and fun to read.
Then the book slows down. There are many interesting places and peoples, but some chapters are tedious. I cannot tell you what happens, because it would ruin the ending for you, but the last few chapters are wonderful, and they make it worth the effort to slog through that middle section. That is, if you’re an adult. The book is, in some ways, about growing up, and I think only the adult reader will be able to appreciate the second half properly.
Of course, Ende wrote in German, so I read an English translation out of necessity. There is only one translation that I know of (by Ralph Manheim). I cannot say if it’s a good translation, or even what a good translation is. All I know is that I enjoyed it. Recommended.