What makes it great? It satisfies the three requirements: interesting, informative, and useful. It's well-written and tells a story, which makes it interesting. It contains lots of information about wines under screwcap and cork, so it's not fluff. And if you drink wine, it's actually useful to you when selecting wines.
It's not trying to sell you on Tablas Creek wines, although they are clearly part of the story.
What it really sells is how fundamentally passionate Jason & Co. are about the wines they produce.