I am changing a 120v receptacle in an old house, have found that there is no ground wire. The receptacle has two black and two white wires, each of which is on the same screw of the old receptacle, though the book I'm using seems to say they should be on different screws. Can I safely instal a new, grounded receptacle into this socket even though there is no ground wire, and should I double up the wires like the old receptacle, or is the book I'm reading correct? Is this an example of bad wiring, or was there actually a reason they would be on the same screw? Thanks for any help.

It is not an example of bad wiring, it is an example of old wiring.

Are the wires inside of a metal box and the metal box connected to conduit?

If they are, then the metal box and the conduit are supplying ground.

To answer your quest regarding the receptacle, no you cannot install a grounded receptacle and have it be up to code.

You have two choices, replace the old wiring with current 14/2 AWG (guessing at a 15 amp circuit) for the entire circuit, or purchasing 15 amp GFCI receptacles and labeling the faceplate to say that there is "No ground wire installed" on each receptacle you replace.