I read the following sentence in a leading magazine -
"If it is to secure its long-term relationship with Iraq, the superpower had better adjust fast to the idea that Iraq is once again a sovereign country, one that has a powerful sense of wounded pride and some prickly sensibilities."
Is it a correct usage?Shouldn't it be "superpower should better" ?I have not come across this usage in any of the usage guides or dictionaries.Please help.

I'm not sure of the analysis, but it's a very old and still common usage. "Best" is less often used in place of "better." The better/best thing for you to do is to adjust quickly.

It's a warning sometimes followed by an "or else" statement. You'd better mind your manners or else you're gonna get a whipping.

Another interpretation might be, It is / would be better if / that you take my advice.

I agree that "should better" would appear to be the correct version, but it's never used.

Best regards, - A.

Edit. Here's a Google reference: http://www.englishpage.com/modals/hadbetter.html
Thanks a lot Avangi for your reply.