The Power of Now is about living in the present moment so you suffer less and have more inner peace. It's really about reconnecting to your physical senses rather than being lost in thinking. Eckhart Tolle says past and future are only mental simulations because your life always happens Now.
The Subtle Art of Not Giving a F*** is like your smart but impolite friend explaining some great lessons from philosophy. Mark Manson shows how to live by your values so you can act with less hesitation, do what's most important to you, and stop worrying what people think.
Man's Search for Meaning was written after Viktor Frankl survived the concentration camps of WW2. He noticed that his fellow prisoners who could find purpose and meaning in their suffering found the strength to survive, while others perished. This book is also a guide to finding meaning in your life.