Pronunciation of 谁/誰 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 (sh&#233;i / shu&#237;) 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.
Quote from “谁”字注音规范化探讨（现代语文(语言研究版), Modern Chinese, 2007年11期 ）


  摘要："谁"在现实生活中应用十分广泛,可就是这样一个常用字,人们有时读"shuí",有时读"shéi",即使同一个人,在相同的语言环境中,对"谁"的读音也是模棱两......


This is all what I can see without an account.

And it suggests that even the same person cannot form a fixed pattern as when to use "shui2" or "shei2".

Maybe the full article can provide more information.

As a native, I can say that I don't know either. So the two pronunciations might just be two possible pronunciations of the same character, both widely used, and mostly interchangeable.