First of all, the price. It's quite expensive, and comparing it to other books in the genre (like this german c++ book) I expect more detail when it comes to explaining how to get the software running. The aforementioned book costs less, is three times the size and includes not only a CD with an automated installer, but also detailed tutorials about how to set the IDE, compiler and even some libraries like boost or Qt up. "I'm sorry we can't tell you how to do this, but I'm sure you'll find some tutorials on the internet" just doesn't cut it.
Not sure what prices you're comparing here. SFML Game Development costs €20.99 as eBook (half that with the promo code on the SFML website) and €34.99 with print + eBook and they have some sales all the time. While the book you linked costs €39.99. How is that half the price or even cheaper?
Don't forget, that the authors are not the ones setting the price, it's all in the publisher's hands.
As for the size, a book doesn't become better the more pages it has, it can actually be the other way around as well, i.e. just filling pages to have a lot of pages.
And then you can't compare a book that talks about a specific library and a book that generally talks about C++. The goal of the SFML Game Development books is not to teach you how to setup your IDE and compiler, but to teach you how to use the library. The book assumes you already know these things and points to the website where we have guides that are kept up to date, compared to a book that might not be up to date after a few months.
Concerning the content, ...
I agree that the explenations aren't always the best. I think it's also important to note that the book was written by three different authors (none of them native English speakers), so everyone has their own style of explaining things.
Lastly, I bought this book so I could learn the basics about game programming. In my opinion, saying "we can't tell you anything at all about GLSL because it's too complicated", "Steering behaviors are very interesting. Don't hesitate to search the internet" or "Separating Axis Theorom is a popular algorithm, read more about it here: ..." collides with the purpose of this book. Why not even try to explain at least a little bit? I bought the book so I could learn about this. I don't need a full explanation, just the basics and a few examples would suffice. I don't know the first thing about GLSL, but the .frag files enclosed are just a few lines long and don't look too complicated.
The book really just focuses on teaching SFML. It's not there to explain all the wonders of the world, however they provide additional references or terms to search for, so you can further study these topics.
I don't know much about the process, but I know, that there has been a page limit and a tight release schedule. With that in mind, it also may make some more sense why the authors just couldn't go very deep on all the various topics.
And finally Packt is a horrible publisher. They chose quantity over quality, so they don't really have proof readers, they don't invest any time in actually editing the final drafts, e.g. trying to make it more uniform or think about the depth of content. Their process is to find some poor soul that wants to write a book, put a price tag on the written pages - doesn't matter if it's good or bad - and thus make profit off it.
Nice to hear that you still like it, besides it not fully meeting your expectations.