It is an incredibly dense book. If you don't like reading history, you won't like this book. The stories in it are thrilling and epic, but they are buried under a wealth. Tolkien created a complete world with The Hobbitt and The Lord of the Rings. This book explains how that world came about. It has semantic history, history of the races' eschatology, geographic development, etc.. I think Tolkien felt compelled to create this book in order to tidy up the Middle Earth.
Tolkien had an amazing mind. Can you imagine creating an entire world that millions of readers will get lost in for decades and decades to come?