SPOILER FREE REVIEW
I absolutely loved reading the book, visualizing each sentence and each scene, and imagining each moment play out as it would have in the game. This is something that the game deserved and would have surely changed a lot of people's opinions about FFXV.
Just the base game is maybe a 7 at best with most of the points coming from the gameplay and the graphical presentation, but with the novel the game turns to an easy 9 or 9+ based on the merits and the epic scale of the story. Must read for all fans who felt the story resonated with them at any level whatsoever in the game and who saw it through to the end.
Read it last, after the DLC episodes available and the other media (movie, anime, etc) and try to play the FFXV soundtrack in the background while you read the last chapter of the book. Really glad I gave this book a chance.