D-modules
D-modules form the essense of the geometric (=automorphic side) of the Langlands correspondence. Formally, D-modules are easy to define; however, your understanding will be very shallow unless you know what D-modules really are, say on usual schemes. As you go deeper into the theory, you also understand why it is so attractive: D-modules are very rich!
Here are some suggestions as to the reading for the general theory of D-modules. They all cover more or less all the necessary material, so you can choose one that you like the most, and go with it.
Here are two "official" books:
"Algebraic D-modules" by A. Borel et al.
"D-modules, Perverse sheaves and Representation Theory", by R. Hotta, K. Takeuchi and T. Tanisaki.
Once we have the theory of D-modules on schemes, we can extend it to arbitrary prestacks. This will be done in the (technically crucial) Talk 1.2. However, it may be a good idea to face it with some prior knowledge of how this is done. Here are some references:
For D-modules on prestacks: Sections 5 and 6 of Drinfeld-Gaitsgory
For the crystalline approach see Gaitsgory-Rozenblyum
.