Bentley makes some good manuals, though I'm not sure if they do em for American cars
As far as your problem with mileage, if you've never had it done, replace your air filter! I'm not sure if your car has a distributor rotor/cap, but if so, do that too - all of these are fairly cheap and should be easy to do with a screwdriver and maybe a wrench or ratchet.
As far as changing plugs goes, they're really easy. Pretty much all you have to do is grab a ratchet and socket (there are only two sizes for plugs - 5/8 and 13/16 in), go to the local autozone and ask em for plugs for your car... I prefer bosch platinum plugs - you don't really need the 2 or 4 prong ones, and they come pre-gapped (though it never hurts to check the gap with a simple tool also available there and the guide for gap size in your manual or on the bottom of the hood). If your cars plugs are pretty tucked away, it can be helpful to buy a wobbly socket and perhaps an extension, while you're at the car parts store.
Once you're in there, all you've got to do is pull the plug wire off, take the old plug out, pop the included washer on the new plug if necessary, put the new one in with light-moderate torque (try to pay attention to how hard they are to get out, which shouldn't be too hard, as many heads are aluminum. Tighten until you encounter resistance, and go just over that. I'd call it just beyond hand tight, with your hand near the head of the ratchet.), and push the wire back on. If your wires are very dry, brittle, or cracked, look into a new set of those as well.
Also, put fuel injector cleaner through your tank every oil change or two. Techron is a good cleaner as mentioned, though you may opt for Redline if you think they're
seriously dirty. Check out the section at your local auto store, and get something midrange, maybe by Gunk, STP, or Techron.
Hope things look up for ya, and you can pawn the car off on some other guy!