The pronoun 谁 has two pronunciations — shéi and shuí — without a difference in meaning. When I started learning Chinese, I learnt the pronunciation shéi. However, one of my Chinese language partners (from Henan) said that he had never learnt this pronunciation and said that no other words in Chinese are pronounced as shéi (so this pronunciation was an anomaly).
(I have also noticed that New Practical Chinese Reader, published by Beijing Language and Culture University Press, uses shéi, while another book Chinesisch für Deutsche from Haiwen in Shanghai consistently uses shuí.)
Are these two alternative pronunciations the result of regional differences? Is one of them the "official" pronunciation?
Update: The original version of this question claimed that the final "-éi" (i.e. with the second tone) does not occur in other Chinese words. As 倪阔乐 pointed out in a comment, this is not correct.