Looking for a textbook introduction to integrals of the following form

Summary:: Pretty sure they have something to do with path integrals, or what not. But obviously it's hard to *search* for this stuff.

Basically, I'm looking for a textbook, any textbook--physics, mathematics, etc.--that deals with integrals that look something like this (mistakes are mine):

$$S = \int dx^4 \Omega \, e^{i \int \mathcal{L} dt}$$

Where $S$ is an action to be minimized, $\Sigma$ is just something integrable across the 4-volume and $\mathcal{L}$ is a Lagrangian. Ideally, looking for something that:

1. explains why the Lagrangian is in the exponent of $e$ like that and what it signifies, and
2. works an example of minimizing $S$.

Basically, just want to know where to start.

Okay, so first of all it seems the action is *not* the expression, but just the integral in the exponent of $e$. That's good to know.