‘Successful Manager’s Handbook’ – it’s only aimed at the manager that is reading it.
Hey, can I get a copy for my boss?
So you believe that the lack of a definite article, e.g. ‘The’ means that manager is singular?
I don’t know the right answer and I’ve asked this question of dozens of people including English teachers and professors, lawyers and writers, even Radio 4!
I get a 50/50 split between the two options; some believe that the lack of a definite article mean that ‘managers’ must be plural, thus it must be ‘Successful Managers’ Handbook’, others suggest as you’ve done; it’s singular and therefore ‘Manager’s’
You can buy the book but it’s pricey and I doubt if your manager needs it that he/she is going to be receptive to reading it. Need and awareness in these issues are usually strongly and negatively correlated!