When does a question fall into "allowable recommendation" category (good subjective) and when does it not?

Question

I only saw the "Good Subjective, Bad Subjective" warning next to book recommendation questions. Do they form an exception? For example this question is a "recommendation"-kind, which is indeed not a good fit for Q&A, but then again, so aren't book recommendation questions (you can't really select a single good answer).

Answer 1

The key here is that we don't want a list of everyone's favourite book. What we want is the question to bring out the book on a given topic.

This means that the the question has to be specific and to the point so:

What's the best book for networking?

would be closed whereas:

What book will help me with X?

where "X" is something specific and clear cut.

Answer 2

ask us what you need to learn to tell what you should buy...

To me, the most enlightening guidance on asking and answering recommendation type questions appears to be one given in Q&A is Hard, Let’s Go Shopping!

...consider the voluminous amount of information you need to even begin properly answering...

Let’s say the question asker provided all that information. Fat chance, I know, but let’s pretend for a moment they did — and we were able to provide the perfect, ideal shopping recommendation to them. Even if that was the case, technology moves so rapidly that the best shopping recommendations will be utterly obsolete within a year! What’s the point of a bunch of labor intensive questions that provide only temporary benefit to a limited (some might say Too Localized) audience? There isn’t any...

... there is a way to ask these questions that avoids the inherent problems with shopping recommendations. For example, let’s say you wanted — as I did — to buy a point-and-shoot camera that takes good low light photos. So we’re going to ask on photo.stackexchange.com, naturally!

Here’s one way to ask:

Q: What’s the best low light point-and-shoot camera?

...Here’s another way to ask:

Q: How do I tell which point-and-shoot cameras take good low light photos?

...The former question provides the path of least resistance: a laundry list of products I can buy without thinking about it too much. But that answer will only be valid for a year at best. The latter question may take some thinking, but its answer will be valid forever … or at least until camera technology somehow shifts beyond lenses and sensors as we know them today. Thus, when it comes to shopping questions, don’t ask us what you should buy — ask us what you need to learn to tell what you should buy...

Answer 3

We welcome some book recommendations, as described in the accepted answer on "Are book recommendations on-topic?" (emphasis mine):

I would say that like "what language should I learn" questions, questions that effectively ask for a list of books on X should be closed as "not constructive".

However, like programming language questions, if you have specialised requirements (you want to go into LINQ, or networking, or.. in great depth) then perhaps those should be allowed. They should inspire answers that explain why a book is good for this topic and shouldn't just produce a list of everyone's favourite.

On book questions that we feel are borderline there are two equally valid options:

1. Add the "good subjective - bad subjective" warning and hope for the best,
2. Close the question as not constructive.

If the question is getting down / close votes, flags, or very bad answers (think: just a link), option 2 is the better choice. The process is about the same for every borderline question and it's always a judgement call.