There have been a ton of book recommendation questions, some fairly old with helpful answers, flagged as not constructive today. According to the last discussion on this, most of these qualify as specialized enough to remain on topic, in my opinion. Has that policy changed? How are people handling those flags? Is there any explanation for the sudden burst?
According to the last discussion, most of these don't meet our quality guidelines (emphasis mine):
After the discussion on meta, we started to apply the litmus test to incoming questions, but we have a huge backlog of book recommendation questions that aren't specialized ("What's a good book for C?", "How can I learn Java?", "Is there a book that'll make me a good programmer?", etc.) and a lot of book recommendation questions that didn't get any love and/or didn't produce explanatory answers:
Because there are other problems we wanted to focus on first, the backlog hasn't been brought up on meta, but someone is taking the initiative to go through and review those questions (for which we are grateful: if you see something, say something and all that).
Having evaluated the flags that have come in, I'd say it's about 9/10 are just terrible book recommendation questions. There is a small number of flagged posts that meet the basic requirements set out in the meta discussion question you linked, and have been revised to be clearer.
Since we're on the subject anyway....
If you want to help review and get these questions up to shape, here's what I've been doing:
Determine if the question is about a specialized or niche topic in program
If the subject of the recommendation is covered in hundreds of online tutorials, thousands of books, or is likely a topic every programmer has been exposed to, it's too general. They're not going to produce a canonical book, they're going to create a small subset of the list of books about the subject.
That is, when a person asks "What's a good book on Java?" and it attracts 5 answers, we're not doing the internet a solid. There are way more than 5 books on Java that are passable, and I think we all know this. These aren't the type of questions we want here.
But if a person asks about a niche topic and it gets a few answers, that's valuable: if there have only been a dozen people writing about the subject, we have a high chance of capturing what is the canon for the subject. That's the type of recommendations we want here.
Clean up the question to invite and demand the type of answers we want
Even if a recommendation question is about a niche topic, a badly worded question can doom it. What I've seen is a question that essentially asks for a book recommendation for a niche topic but clouds that question with things like:
This is not good. In our FAQ, there are several guidelines our questions need to meet:
So what I've done on the questions that are about niche and specialized topics is rewrite it to follow a set template:
And tagging it with books and the subject of the recommendation. This way, we can transform the book tag from being a catch-all for recommendations that wildly vary in quality and scope to a really great library, where you can just search for the topic you want and find a list of books for that niche topic you need to know more about.