Any doctrine should be like a living being. It has its origins and then develops in many different ways. One should not take its founders to the letter, in a Koran-like fashion. But, likewise we must require that its many emanations have a minimum content from the founders, the same way a burger to be a burger must have a minimum of beef.
Therefore, finding a fine balance between innovation and tradition should be a never ending pursuit for a wise person. Its first step must be reading some of the founders’ writings.
For those in a hurry, Wikipedia has an entry with a good overview about liberal and liberalism. There you can read that its ancestry goes back four hundred years to the beginning of the English Civil War. With the rise of the Enlightenment, the word acquired a more positive undertone, being defined as "free from narrow prejudice" in 1781. However, the word liberalism only began to be used around 1815, while the Napoleon troops ravaged Europe.
For those with time to follow the development of the liberal doctrines here is a short bibliography of the main classic texts, all of them available in a Kindle Edition at a low price.
John Locke’s Second Treatise on Government (1689) is a good start to understand why people have natural rights and the main purpose of governments is to safeguard such rights. Likewise Adam Smith’s Wealth of Nations (1776) provides the basis to understand the role of free markets.
Jeremy Bentham’s An Introduction to the Principles of Morals and Legislation (1781) launched the basis of utilitarianism while John Stuart Mill’s On Liberty (1859) defended the right of individuals to control their own mind and body, guaranteed by limiting the exercise of power to prevent individuals from harming others.
Then the twentieth century brought in a divide about how far the government should go. Now, we have the minimalist and rule based schools (Austrians and Monetarists) and the Keynes-inspired school of discretionary interventionism (in fact most of his followers crossed over to the socialist anti-liberal camp). Here, the two basic references are Friedrich Hayek’s The Road to Serfdom (1944), showing that reliance on free markets would preclude totalitarian control by the state, and John Maynard Keynes’ General Theory (1936) claiming that in special circumstances, when countries were caught in a liquidity trap, state intervention was indispensible to avoid depressions.
Currently, the two branches are frequently confused by identifying them with conservative or progressive parties. By reading the classics one hopes that a renaissance of liberalism in the 21st century will discard such connotations.