Like others I have seen the Downtown show, watched both movies, etc. I am not as passionate about the work as others are but do appreciate a fun, soapy show/movies that have gorgeous people, beautiful costumes, spectacular sets/scenery, etc. If you're familiar with the show and have seen this movie (it's the second of two so far), you won't be too surprised by it.
It's a gorgeous coffee table-of book with fantastic pictures of the cast, scenery, costumes, etc. There are thoughts from various people involved with the production, background on the sets/costumes/props, some historical context, etc. Some of it is probably familiar if you know the show/movies and/or kept up with the release media promotions, etc. I don't think there's much that will surprise.
Not much more to say about it. Maybe it was me but I found it a little dull--maybe because I'm familiar with the work as a whole and/or it's not as something I'm into anymore. I also did not think the second movie was as good as the first (although certainly enjoyable in its own way, since at least part of the cast did get to "France", although due to COVID this was a bit of a struggle).
I will say it was interesting to see how the pandemic affected production, so that might be something of interest to historians or academics, but this was not really discussed in depth here (Hugh Bonneville talks a bit about it in his memoir which I read soon before this, so I may be mixing things up a bit). Still, it was all gorgeous to watch and if you're looking for some escapism and enjoyed the show, this probably a movie for you.
Is the book worth it? For the hardcore fans, I'm sure you'll love it. I decided to buy this as a bargain buy (and was further discounted because of the holiday season), but it's probably only for them (which is probably how I got it as a bargain buy). Maybe of interest to film/media historians for the sake of information about the production, etc. If you're just a casual fan, the library borrow (or even just look at it at the library) or clearance if you want it but don't have the time at the moment to read.