‘Successful Manager’s Handbook”
because
“Successful Managers’ Handbook” looks and feels wrong. You could argue that a book is only read by one person at a time, suggesting a singular reader, even if there are several managers the book might appeal to or be written for. (But I think most writers imagine they are talking to one person when they write.*)
But
“The Handbook of Successful Management”
or
“A Handbook for Successful Managers”
or
“How to be a Successful Manager”
might be better titles for that particular handy-sized book.
* A similar problem arises for tech authors when they have to write something for user’s/users’. Most folk settle on the singular in that case.
I’m bored.