Q:

Is [kernighan-and-ritchie] a valid tag on Stack Overflow?

I'm aware the K&R C is a fairly prevalent C standard, but should kernighan-and-ritchie be a valid tag given that a lot of the questions that use the tag pertain to a specific book?
What is Stack Overflow's policy about book specific tags?

A:

Yes it is a valid tag. The tag is for code and examples from the K&R book, which is (unfortunately) frequently used by beginners still to this day. We created kernighan-and-ritchie exclusively for questions regarding the examples in the book. 
One use for the tag is to explain why the code in the question is strange -  because it is a copy/paste from an obsolete book, rather than the OP being bad. So the tag is rather similar to language-lawyer in a way, as it is used to show that the code is artificial and badly written for a reason.
This is not to be confused with questions regarding pre-standard C, which should use kr-c. (And those question are rare)
There's a meta discussion about it here. Which I believe lead to burnination of knr. kernighan-and-ritchie is supposedly the replacement for that bad tag.
This is all explained in the kr-c tag wiki and the kernighan-and-ritchie tag wiki respectively.
EDIT
I assumed that everyone participating in the discussion actually have a clue about the topic, but apparently not... 
This book acted as industry de facto standard for the C language during the years 1972 to 1989, before a formal standardization was available. So it is not just any random book, it is intimately connected to the language specification itself, with the inventor of the language as co-author. As such, it is absolutely on-topic.

