it's GREAT, but in a wildly different way. The books are very, very different. While TFD is a short, concise, epilogue of sorts, The Secret History is a much longer, rambling exercise in world building. It connects the world of Twin Peaks to everything from Lewis and Clark to Richard Nixon to old UFO conspiracies and the illuminati and Crowley.
So to answer your question, YES, it IS as good. But it's WAY more about mysteries than giving answers.
Ultimately, it's canon. It's written by Mark and features Twin Peaks throughout. And it's also in universe: it's another document from within the twin peaks world.
You personally @
henryeatscereal ? You will fucking LOVE it I think, just as I did. And you MIGHT like it better, just because it's much longer!