Neither fish nor foul, uncategorically other, and perhaps the most abandoned book in the western fantasy canon, The Simarillion (1977) by J.R.R. Tolkien, published posthumously, continues to divide and muddle fans to this day.
When I mention this book to many fantasy readers, their response is usually, "I couldn't finish it." This book, this collection of stories, provides a detailed background and mythology to Middle Earth. Part history, part short story, part epic poem, part legend, part myth, the collection resembles nothing published before or since. For the reader, it either provides more of what they so desire, the details of Middle Earth, or provides what bores them to tears, the details of Middle Earth.
I first read this back in high school, 10th grade, and even back then, as earnest as I was, I found the work hard reading. I could toss off anything else, but I could not toss off this. By the end, I confess that I wasn't paying much attention as I pushed towards completion. Decades later, I came back to the work on audiobook, listening through it as I played endlessly with my then kitten, who needed lots and lots of string. I got more of it that time, but I forgot almost as much.
If you're a deep Tolkien fan, this book has what you want. This is distillate from which the literary tales were created. If you aren't a fan, this is exactly the part of the novels that lost you, equally concentrated.
While I appreciate many of the ideas and tales in the book, they lack the engagement of actual stories, the one that most fantasy readers enjoy, where they form attachments to heroes and learn to despise villains. Due to the structure of the tales, there's little to attach to unless it's Middle Earth itself. Because of this, all these stories feel a bit remote, far away, uncompelling unless its Middle Earth itself that you love.
Even though this book was published in the 1970s, it wasn't a product of the 1970s. In every way, this is an older work salvaged from previous decades, making the best of assorted and often inconsistent source materials.
In my most recent attempt to reread it, The Silmarillion escaped me. There are many who can sing their praises of this work, but I am not one of them.