“Had better for Would better. This is not a defensible as an idiom, as those who always used it before their attention was directed to it take the trouble to point out. It comes of such contractions as he’d for he would, I’d for I would. These clipped words are erroneously restored as ‘he had,’ ‘I had.’ So we have such monstrosities as ‘He had better beware,’ ‘I had better go.’”
Excerpted from: Bierce, Ambrose. Write it Right: A Little Blacklist of Literary Faults. Mineola, NY: Dover, 2010.