The Curmudgeon's Guide to Getting Ahead by Charles A Murray
Murray's prescriptivism falls too narrow; much of this advice will be unintelligible within the century.
A better reduction is as follows:
Murray actually recommends watching Groundhog Day at least three times.
Kill your darlings.
One of the glories of the English language is that it has vocabulary that can be called upon for all those situations. The heedless younger generation has frittered away that patrimony.
think of your twenties as a time for doing the things that you wont be able to do when you have a spouse and children. There is only the stipulation from the previous tip: You have to support yourself. An essential part of the experience is being on your own.
The most famous is Herbert Simons calculation that to become an expert requires mastering enough chunks of material relevant to your field. The number of chunks is usually described as fifty thousand, mastery of which usually requires at least ten years.