What is the relationship between The Spirit of the Laws and The Social Contract?

These two works are similar in that they are major works
of political thought, but they are not really trying to address the same issue. 
Rousseau's work is trying to look at the bases for society -- why society comes to
exist.  Montesquieu, by contrast, is trying for a more applied approach.  He is trying
to look at how a society (once it is created) should be
governed.

Rousseau was concerned with why societies form. 
He was thinking about the state of nature and how society forms as a response to this
state.  Because of this, he was laying down only the most basic ideas about the actual
governing of a society.

Montesquieu, on the other hand, was
trying to look at the political institutions that societies have.  He was trying to show
which political institutions were best.  This is a more applied book -- he is looking at
the actual workings of government as opposed to the theories of why governments
exist.