One of the reasons why I enjoyed If On A Winter's Night A Traveller was that it just felt like Calvino had a lot of fun writing it. It's such a delightful novel, and the unconventional structure allows for experimentation and creativity. Also, being a meta-novel about books allowed him to express his love of books.
A lot of fantasy novels with elaborate worldbuilding often feel like an author's genuine passion project as well. Lord of the Rings is the obvious example here! I also love the way China Mieville writes his worlds, they're so lively and intricate. You can tell he really loves writing about every nook and cranny.
by moss42069