I read in a book that for comparison,
as ... as is used for affirmative statements, and
so ... as is used to for negative statements. For example:
He is as tall as his brother.
He is not so tall as his father.
I did not find any other sources where such a "rule" is either confirmed or denied. Personally, the construct
not so ... as sounds rather odd, and there doesn't seem to be anything wrong with
not as ... as.
What is the correct "rule" in such cases, if there is one?