I love coffee flavoured anything! Funnily enough don't drink a lot of it!
I would go the espresso powder route (as BL says its very very fine) and as to quantity, well just keep tasting until you are satisfied. I think liquid coffee would adversely affect the batter...I think the camp coffee thingee would work, have seen it as tesco's but never thought to buy! If its shortbread sorta biccie, then assuming you are going to cream the butter and sugar and if so, perhaps add the coffee after that, mix and then flour etc? Am not a natural biscuit maker so am guessing here..
On the subject, have you tried the splodge biscuits in HTBDG? I love love love those and they are somewhere between a cake and a biscuit...it has walnuts in it which i don't always add...and everyone I have served it to laps it up...
Ermm..if this works..and you are happy with it...ermm post recipe??? puhhhhlllleeeeaaaaassseeee???