What was Baron de Montesquieu "the spirit of laws" book about?

The Spirit of Laws is an extensive work written by Baron de Montesquieu in 1748 that covers many areas of law and compares the provisions of law in force in different countries in different eras. However, the work is best known for the fact that in it Montesquieu presents his doctrine of power-sharing, which has played a very important role in the development of forms of government in Western democracies. Montesquieu considered the concentration of power to be the worst threat to civil liberties. His solution to this threat is the doctrine of the threefold division of power, according to which the legislature, the executive, and the judiciary had to be in the hands of different people.