Plug cuts for fun and profit
Reading plugs, aka plug cuts, is a good way to find out what is going on inside your engine. Some good references are old Circle Trace+Racing Technology magazines, Smokey Yunick did a 2 part series. "The Doctor's Step-By Step Guide to Optimizing Your Ignition", ISBN 0965085600, has a chapter on plug cuts.
What you'll need:

Long empty road
Spark plug socket
Magnifying glass
Spare plugs
What to do:

Start with plugs that have some use, you want them to be a bone white color
Make a WOT run or 2 in 3rd/4th gear. You want to load the engine as much as possible and you can't do that in a low gear. Pull over and kill the engine as soon as possible after the run. Please be careful killing the engine while still moving, you'll lose power steering, brakes, and possibly lock the steering column.
Remove plugs, examine with magnifying glass.
KEEP A NOTEBOOK. Record results and atmospheric conditions (pressure, temp, humidity).
Indexing plugs can help make them easier to read. Generally you want the most open part of the plug exposed to the intake valve. Indexing means using shim washers of varying thicknesses on the plug such that when tightened, they're all in the same orientation relative to the valves. Put a reference mark on the outside of the plug before installing so you can tell where it's pointing when torqued.
Here are some conditions to look for: White leaning to beige Good WOT mixture
White leaning toward black Momentary rich spot
Bonding agent cooked out between porcelain and center electrode, or the side electrode gets a metallic green/blue hue Plug range too hot, or temp too high in combustion chamber, possibly too much timing
Heavy black flakey deposits Too much fuel
Black glossy deposits Oil
salt and pepper flecks/fly turds/tiny metallic balls on porcelain Detonation. The flecks are little bits of piston
Fire ring (tan spot/ring) If high up on porcelain timing is probably good. If it's near the tip you might want to try a little more timing
Corners rounded off center electrode (newish plugs) lean/too much timing. Or just normal wear if the plugs are old
Black spiderweb traces on center electrode Usually only on early HEI systems, a better spark box or maybe low resistance center button will fix it.
Pools/spots of odd colors Fuel splashing, fuel additives. Can vary with brand. Manganese leavs a pink residue which can be a ground path and cause misfires.

Some notes:
Believe what the plugs tell you, and don't try to force your ideas of what timing, etc. should be onto the engine. Just give the engine whatever it wants.

Sometimes a bad combination can give an OK looking plug (ie thermostat too cold, too much fuel, and too hot plug).

Usually on street motors you'll find one or two cylinders that read a little lean, or are a little more prone to detonation. These always need to be checked. Occasionally do them all. And when roughing in an initial tune, check all of them until you are getting close.

--Bruce Plecan