What is the relation between the two pronunciations shéi and shuí of 谁?
Ross and Ma Modern Mandarin Chinese Grammar seems not to mention shuí, while two different series of readers with pinyin and characters both use shuí excessively.
On line I find every possible answer: some say it is a matter of taste, some say it is regional, some say literary versus colloquial/
In the question Are the two pronunciations of 谁 (shéi / shuí) the result of regional differences? , moderator songyuan expresses an opinion that shéi might be more common in speech. Besides that moderator songyuan labels this "IMO," rather than as fact, it does not even say whether the pronunciation is regional or not. And regionalism is not the only possible relation between the pronunciations.