Aside from the outdated (the book was written in 1902) and cringe-worthy use of the word "man" to refer to all of human kind, this short book is a good reset button for the mind. Something to bring your thoughts into alignment with your goals.
The basic message is that your inner thoughts shape your outer world and circumstances. Of course this concept can be extrapolated to form an outlook that ranges from practical and disciplined to wacky and mystical - but the core idea is valuable in optimizing your mental patterns to match your goals.
Here are a few of my favorite quotes:
"Until thought is linked with purpose there is no intelligent accomplishment. With the majority the bark of thought is allowed to drift upon the ocean of life. Aimlessness is a vice, and such drifting must not continue for him who would steer clear of catastrophe and destruction.
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A man should conceive of a legitimate purpose in his heart, and set out to accomplish it. He should make this purpose the centralizing point of his thoughts...he should steadily focus his thought-forces upon the object which he has set before him. He should make this purpose his supreme duty, and should devote himself to its attainment, not allowing his thoughts to wander away into ephemeral fancies, longings, and imaginings. This is the royal road to self-control and true concentration of thought. Even if he fails again and again to accomplish his purpose (as he necessarily must until weakness is overcome), the strength of character gained will be the measure of his true success, and this will form a new starting-point for future power and triumph."
"Thoughts of doubt and fear never accomplished anything, and never can. They always lead to failure. Purpose, energy, power to do, and all strong thoughts cease when doubt and fear creep in."
"Men do not attract that which they want, but that which they are. Their whims, fancies, and ambitions are thwarted at every step, but their inmost thoughts and desires are fed with their own food, be it foul and clean".