If you have the time to read these books on democracy, you will have drawn up a broad panorama of what is democracy and what the men worked out behind this concept. Indeed, it is these books that have forged and best present the notion of democracy.
In order of preference :
1. The Politics – Aristotle (buy the paper version): Aristotle makes a typology of the different state regimes. Very interesting to understand how the Greeks perceived the State and its different forms, in particular on democracy (which was not Aristotle’s preference).
2. Du contrat social — Jean-Jacques Rousseau (purchase paper version or) : a must when it comes to democracy! All our democracies are more or less based on this Jean-Jacques Rousseauist idea of the social contract; the book is not only interesting and well written, but it can be read very quickly, and costs only 2 euros in the paper version.
3. On Democracy in America – Tocqueville: this is the greatest and most relevant study ever undertaken on democracy. In addition to being a founder of sociology, Tocqueville tackles a still very modern democracy: the United States, and in very extensive pages, so much so that there are two volumes: choose the most interesting passages, on democracy explicitly, but also on religion, on men, which all feed the reflection on democracy.
The Republic – Plato (Books 8 and 9 in particular) (buy the paper version): a discussion on the best state regime and on what tyranny is by opposition
The Open Society and its Enemies, Karl Popper (buy the paper version): a bit long, you can profitably skip the pages that concern the critique of historicism, and read it instead for its defense of democracy, and the definition he gives of it.