Savings and index fund investing. I have an automatic bank transfer to my broker account every month and then I'm using Interactive Brokers API in a script that buys global equities index ETF.
I can't give just one, but here are a few that had a strong influence on me:
"12 Rules for Life" by Jordan Peterson, "Why Switzerland?" Jonathan Steinberg, "Little Book of Common Sense Investing" by John C. Bogle, "A Random Walk Down Wall Street" by Butron Malkiel, "Liberalismus" by Ludwig von Mises, "Meditations" by Marcus Aurelius, "A Treatise of Human Nature" by David Hume, "Thinking, Fast and Slow" by Daniel Kahneman, "The Law" by Frédéric Bastiat, "Autobiography" by Benjamin Franklin, "Common Sense" by Thomas Paine, "Gespräche mit Goethe" by Johann Peter Eckermann, "Walden" by Henry David Thoreau, "The Old Regime and the Revolution" Alexis de Tocqueville, "On Liberty" by John Stuart Mill, "A Treatise on Political Economy" by Jean-Baptiste Say, "The Man Versus the State" by Herbert Spencer, "The Revolt of the Masses" by José Ortega y Gasset, "Capitalism, Socialism and Democracy" by Joseph Alois Schumpeter, "The Logic of Scientific Discovery" by Karl Popper, "The Machinery of Freedom" by David Friedman, "On Power" Bertrand de Jouvenel, "1984" by George Orwell, "The State" by Anthony de Jasay, "Sketched With the Quill" by Andrzej Bobkowski, "My Correct Views on Everything" by Leszek Kolakowski, "The Captive Mind" Czesław Miłosz, "One Day in the Life of Ivan Denisovich" by Aleksandr Solzhenitsyn, "The House of the Dead" by Fyodor Dostoyevsky, "Conversations with an Executioner" by Kazimierz Moczarski, "Diary 1954" by Leopold Tyrmand, "The Master and Margarita" by Mikhail Bulgakov, "A World Apart: A Memoir of the Gulag" by Gustaw Herling-Grudziński
Open a cheap investment account (e.g. Interactive Brokers) and invest in cheap ETFs (e.g. Vanguard Total World Stock and/or Vanguard Total Bond Market - in proportions 70%/30%) until you have 25*your yearly expenses (that usually means 10-12 years of investing, when you save 50% of your salary). After that point, you're FI, you can withdraw (max) 4% a year and live off from returns from your investments ad infinitum.