A good black powder solvent, patches, wiping rod, blow tube, and of course black powder loaded cartridges. I also have a bullet seating device for those occasional cartridges that need a little extra push to seat into the chamber so the block will close.
I am no expert by any means and just shoot for fun and occasionally hunt.
As previously mentioned the suggested books will supply you with all the excellent information you need and then some as well as the Shiloh forum.