Instrument stuff for sure (basically make sure you're current on flying IFR and your associated knowledge), and maybe reference high altitude aerodynamics a bit. It helped me to know the basics of a turbofan too (it had been awhile since my college courses on the jet stuff). Anything pertaining to the aircraft itself or airline procedures, avoid -- you'll confuse yourself and won't know which airplane you're getting anyway. I know everyone says this, but it's true -- everything you need to know they'll teach you. Except for the things I suggested taking a look at if you want to -- it's assumed you know instrument stuff, it's assumed you know why you can only climb to certain altitudes or why it switches to Mach at a certain point, and it's assumed you know about turbine engines to some degree. Enjoy the calm before the storm-- don't look for more to stress about--relax!