"Lord Valentine's Castle" is a 1979 epic science-fiction/fantasy novel by Robert Silverberg, detailing a political struggle in a far-future setting. In my edition, it is about 450 pages, and it had a series of sequels.
A word should be said about this book's genre, because it sets the tone of the book. This book is nominally a science-fiction novel, set on the world of Majipoor, settled by colonists from earth and other planets around 10,000 years before the book takes place. But in practice, this book reads more like a fantasy novel, because the planet has a feudal setting, with hereditary royalty, and with magic being normal. (Maybe the "sorcerers" are just sufficiently advanced technology, but if so, it is never explained). Almost all of the book makes sense as a chivalric romance, not as a science-fiction story.
The story begins when Valentine stumbles into a small port town on the edge of the planetary empire. He has lost his memory, and has to have a young herdsman explain to him basic things like how money works. Although dazed, he is intelligent, and when he meets a band of jugglers, he is invited to join their juggling band. He then starts travelling across Majipoor with them, but there is a problem: the new "Coronal" (King) of the planet is also named Valentine, and our memoryless travelling juggler starts having flashes of insight. Which, we learn fairly quickly, is because he is actually Lord Valentine, displaced into another body while an usurper has possessed his own body. We then follow an unlikely quest, as Lord Valentine and his crew of jugglers travel across the world, gaining more followers and regaining his memory. Along the way, they encounter a string of adventures, such as riding rafts down whitewater rapids, being swallowed by a gigantic sea dragon, and being entrapped by man-eating plants. This all leads to the book's conclusion, where the dynastic struggle is concluded---but a new mystery is opened.
One thing that is most remarkable about this book is how light it is, especially for modern audiences whose introduction to dynastic politics in epic fantasy is the violence of the Game of Thrones series. In the book, it is said there has not been full-scale war on Majipoor for thousands of years. Lord Valentine tries to reclaim his throne with a minimum of violence, and this is helped by the fact that he has a power that works like a carebear stare: he literally just focuses feelings of love on his enemies and they switch sides. Rather than being a war story, this is an adventure story, and Silverberg seems to enjoy describing the varied geography of Majipoor as much as he enjoys moving the plot along. Some of the flights of fancy, in describing both people and places, seem like they belong more to Dr. Seuss than to "serious" fantasy: the book even introduces two-headed aliens.
The book actually made me think more about something I have been considering more in fantasy lately: how fantasy runs the gamut from the purely "zany" works such as Alice in Wonderland and The Wizard of Oz, through fantasy that is basically history with dragons added (such as, of course, Game of Thrones). A fantasy novel that allows its background to be painted in vague terms, that is a bit malleable with its biology, physics and basic ontology, allows the writer to do whatever he wants, skipping over questions about (as George RR Martin asked:) "What was Aragorn's tax policy". It allows a writer to do whatever he wants and to bring forth a number of amusing notions, and to have an adventure where morality and humanity wins in a way that it couldn't in the real world. The problem with this is that when anything is possible, there are fewer risks. I like this books moral and adventuresome tone, but I do admit that by the end, this light tone made the ending seem anti-climactic. And yet, after finishing this book, I also want to read the sequel.