Thoughts about the book(s) and the movie - some spoilers
I've read all three books of the series: "Sidways", "Vertical" and "Sideways 3: Chile"
The first book and the movie that came out of it are both very very good. Big fan of both. In the core the movie stayed true to the book. But in the book a lot more happens, which had to be cut down for the movie of course, it's already a two-hour movie with the cuts. Some of the extra adventures in the books are quite funny and other parts I'm glad they left out.
The movie adaption has a few points which I like a lot more than the way the book handled it. Firstly: Maya. In the book she gets paid by Jack to hook up with Miles. In the movie it's sweeter, more natural...and without the being-paid part. Felt a lot better that way. Stephanies character was named "Terra" in the book and was quite more over-the-top. They toned her down for the movie. Another wise decision. The ending of the movie is also - let's quote Jack - "vastly superior to the original ending". In the novel Miles and Maya did finally hook up. The ending in the movie with Miles knocking on Mayas door was perfect in my opinion.
So much about the first book and the movie. Let's have a look at the sequel "Vertical". Rex Pickett, the author, was clearly influenced by the movie and obviously not too happy about it. Especially about how Miles was portrayed in the movie. It's even stated in the novel, that the "Movie-Miles" is nothing like the real Miles (in the novel the movie exists - based on the character Miles). And then the author completely changed the whole Miles character. He felt nothing like in the first book. Author clearly upset with the movie in my opinion. And that's where the second book failed for me: The characters felt nothing like the ones we liked and loved from Sideways. A complete 180°. Story-wise it got a lot more dramatic and serious in the second half. Just tragic without having any fun part.
Obviously I wasn't the only one feeling that way. Because then the second sequel came out with the really cheap and unfortunate title "Sideways 3: Chile". A nice location with more interesting wines for Miles. But the book felt rushed, nothing really happened. Except for one thing: The Miles character was switched back and was written like in the first book. As if "Vertical" never happened. It's a bit confusing and really isn't in canon with the series. Jack makes a quick cameo, but adds nothing to the story. He was already written out halfway through "Vertical". Too bad.
In the end it's only the first book that matters. The other two were ok to read. But uninteresting and forgettable. The biggest complaint about the second I would say is the poor handling of the Miles character. And the biggest complaint about the third book is that it's a rushed production without any real content.
As Miles might say: Sideways is a 61' Cheval Blanc. Vertical and Sideways 3 are Merlot table wines.