Most (if not all) explicitly say they are not to be mistaken (in any way) as legally binding contract. With at will employment, this seeming nuance is worth keeping in mind. It means the employee cannot rely on such a handbook, in court: the employer has explicitly prevented this use. As you correcty point out, an explicit contract may require one or both sides to live up to the handbook. But such employment contracts are not considered the general case, in the US at least.