The book contains plenty of new content, but nothing that was intended as a story
The vast majority of the content in the book is either derived from notes Tolkien made when trying to rework the timeline of the first age (when extending the length of a Valian year to 144 sun years), or from various late philological essays. If you've read the material published in Unfinished Tales and the final three volumes of The History of Middle-earth, you'll understand that this type of stuff can often turn into narrative, but is generally not originally intended as such.
So too in this book. There are certainly texts present that with certain editorial trickery can be made to look like narratives (like was done in Unfinished Tales), but it is all presented as-is (just with some of the more linguistic parts edited out), and so nearly everything in the book will read like essays rather than stories.
Also, as you say you have access to the amazon preview, I'd encourage you to continue a little bit past the table of contents and read the foreword of the book. It explains what the book is and isn't.