I had read the book several times in the early and mid 1980s when I was in high school and college and re-read after many decades.
It is an instructive self-help book and if the author's advice is seriously followed one can develop an informed mind, healthy body, good morals and active civic life.
Lala Hardayal was a rationalist and railed against religion, spirituality and metaphysics. But billions of people find solace and peace in them and it would be overly arrogant to suggest they are misguided fools or blind worshippers. Further one should be skeptical about the claims being made about rationalism itself. Humans are not fully rational and hence some limits in it are inherent.
I also found it jarring that Hardayal regarded ancient Greece and its civilization as closest to being the ideal. He overlooked that it had crumbled to dust and died ages ago, but the ancient civilization of India had a vitality and spirit that persists strongly and touches many aspects of our lives till this day-be it religion, philosophy, medicine, mathematics, eating habits, festivals, yoga, art etc