The basic strategy of value investing is very simple. You find an amazing company, calculate its true value (also known as Intrinsic value) and pay a lot less to purchase the stock (when the market is down or when the stock is on sale in the market).
As you have bought the stock at a discount, you can make a profit by selling the stock when the price reaches its true value.
A few good books to learn value investing principles are The Intelligent Investor by Benjamin Graham, Value Investing and behavioral finance by Parag Parikh, The Little Book of Value Investing by Christopher H. Browne, The Warren Buffett Way by Robert G. Hagstrom, and The Little Book That Beats the Market by Joel Greenblatt.
Further, you can also read the articles by Trade Brains to learn value investing principles.