I have a regular audio cassette, the kind that goes
into your car's regular cassette deck. It's an audio
book that was purchased a few years ago, and I was
hoping I could try to salvage it digitally. Since it was
obviously some manufacturing problem, I tried
contacting the maker but they have not responded.

YET ANOTHER example of asking for assistance and NOT giving the full story.
Shame, shame, shame.

The kind of problem you're describing SHOULD NOT occur with a Compact Cassette,
because the left and right tracks are adjacent, and correctly "sum to mono" when
played on a mono machine.

Several people have suggested severe head misalignment, and I think that's
likely. Try playing the tape on other machines -- both mono and stereo -- to see
what happens. This should reveal whether the problem is with the tape or the
player.