You had a problem with the Dusty Phillips book ranking on top of Powerful Python. And I can personally guarantee that this book is out of the world good and deserves that spot, if not higher (though the book above by Luciano Ramalho is also very high quality). Here is my review of the Dusty Phillips book, in case you are interested [0].
On your last point, it is certainly best to judge a technical book solely by the content. The problem is : you don't have access to the content until you buy the book. You can of course take a look at the TOC, but doing that for all books in a given field would take a lot of time. It is always nice to have some other indicator that correlates well with quality e.g. average reviews or popularity so that you can reduce the problem space. This is what PythonBooks aims to provide.
[0] http://pythonbooks.org/python-3-object-oriented-programming-...
Rarely on top but still a high ranking relatively speaking(2-4 etc.)
>"You had a problem with the Dusty Phillips book ranking on top of Powerful Python."
I didn't have any problem with this book and even mentioned I hadn't read it. I simply used it as an example of a book that had another perhaps more popular retail outlet than Amazon, that's all.
If you look at the beginner book section, the kid's section and the application walkthrough section, there are almost no Packt books there. In the reference book section, the Packt book ranks last among 6 listed books.
In the individual topical sections, there are on an average 5 books listed per topic, so 3-4 is a pretty low rank.
The only two exceptions are the ML book by Sebastian Raschka and the high performance book by Gabriele Lanaro which are ranked in the 2nd position. Both of these are outstanding books in their own domain and deserve their spot.
I see your point about Powerful Python having another retail outlet, but I think the same is true for all the other books because they too have other retail outlets.
This observation about the poor quality of their books and their shady practices has been noted by many others as well. I am not saying all of them are bad but certainly the majority of them are. See these discussions:
https://www.quora.com/Why-are-the-books-from-Packt-Publishin...
https://forum.unity3d.com/threads/so-i-got-contacted-by-pack...
I have not read the particular Python book you are referring to but I have viewed many many other Packt Publishing titles and they are almost always poor and unacceptable quality. The reasons are what I stated earlier about poor English, typos, not spell-checked, bad grammar and often simply little more than paraphrasing official documentation.
I consider publishers like Pragmatic, Oreilly or Manning to be examples of reputable publishers. Packt comes nowhere near this level of quality or professionalism.
>"However, just as HN tries to provide value by best effort quality ranking, PythonBooks also makes a best effort attempt at judging quality. It is not perfect, but it's the best I could do."
I really don't think there's any substitute for judging a technical book solely by the content presented in it.