What is the aim of liberty in The Spirit of the Laws?
In The Spirit of the Laws Montesquieu defines liberty as "the tranquility of mind that results from the conviction that everybody has his security." This indicates that for Montesquieu, no less than the Founding Fathers, liberty could only arise if the appropriate institutions were in place, institutions that guaranteed the citizen's security under the rule of law. But this liberty that Montesquieu prizes so highly is not the right to do whatever we want, whenever we want; Montesquieu doesn't confuse liberty with license. Under the rule of law, liberty resides in "the power to do what one should will, and not to be compelled to do what one should not will to do." In other words, citizens must be allowed to do what they ought to do according to commonly-accepted laws and rules, but not be so compelled by any government institution.
Montesquieu further suggests that the best system of government for preserving liberty is one in which there is a separation of powers between the legislative, executive, and judicial branches. He famously misinterpreted the unwritten British constitution as providing for just such a system. However, it wasn't until the founding of the United States that Montesquieu's ideal system of government, and with it his ideal of liberty, was put into effect.
When Montesquieu talks about liberty in this book, he does not mean it in exactly the same way that we (at least in the US) use the term. For him, political liberty does not mean some moral right that we have to enjoy freedom of speech or the right to vote. Instead, he is referring to the ability to do whatever the laws allow us to do. Montesquieu is not asking about whether the laws are just. Rather, he says that we have liberty whenever we are able to do everything the law says we can do.
The aim of liberty is simply to create a more just society. A society is just if the people have liberty. In that way, liberty is an end in itself -- liberty itself is the goal of society. When we have liberty, we are able to feel safe. We are able to feel that the state will not punish us for no reason.