Modules in a topos

What is a good source for the category of modules over a ring object in a Grothendieck topos? I am particularly interested in the question: if I is an injective module and E is an object of the topos, then there is an exponential I^E. Is it injective? Equivalently, are internal copowers left exact? Michael 30-Aug-2001 10:34:31 -0300,1968;000000000000-00000020