By the rules of exponents

e^{-x-y} = e^{-x} * e^{-y}

so you can write your integral as separate integrations over x and y:

Int[e^{-x-y} dx dy] = Int[e^{-x} dx] * Int[e^{-y} dy]

and

Int[e^{-x} dx] = -e^x (over the appropriate limits)

As I don't know which limits go with which variable I can't help any further, but I suspect you can take it from here.

(I'll let someone else handle the question about references.)

-Dan