About the first thing I got was a skiver blade, it's a shankless blade ground on just one side so you can sliver off the edge of a skin on the inside until it's is almost non-existant: you then glue and sew a little way back from the edge. There's one type of edge creaser which has two blades set by a screw, one runs "in the ditch" alongside the side of the leather and the other presses into it at a constant distance from the edge, creating the recess the stitching marker runs in to leave the stitching buried rather than standing proud of the surface of the leather - it's still visible but won't rub.
As far as decorating leather's concerned, you need to use vegetable tan. To check for this, see if it'll take in water easily - if a drop of water goes straight in, then it's veg tan. You need it to soften the surface for punches and other carving impression tools. Then, for outlining, you'll need a pivot knife for starters, plus a beveller and some pattern stamps - Tandy do these.
If you get into thonging, then thonging stamps are a good idea to get a steady and equal spacing and angle in the slots for the thonging. You can work that up into some fancy Spanish edge braids. Don't use a hole punch the obvious way, reverse it so the punch you want to use is on the outside, place it where you want to make the hole on the leather, resting on some waste wood, and use a hammer on the back of the anvil arm of the punch. They're rarely much use beyond 1mm leather, they won't cut cleanly any thicker if you use them as plier punches, which is why we do it the other way.
