This is Swierczynski's second appearance here.
As I have said before, yea, in this very space, you don't need a new plot device to write a terrific story. You can just think of something original to do with an old one.
The idea of two personalities inhabiting one body goes back at least to Robert Louis Stevenson. And that appears to be what's going on.
Gibbs is keeping a journal to try to make sense of what's going on in his life, and maybe in his head. A woman in California invited him to her party so he driving all the way from Philadelphia for the occasion. He has no clue why he would agree to do that.
But someone else writes in the journal too when Gibbs is drunk, and then maliciously destroys the pages...
Is this a simple case of psychosis or is something much more sinister going on?
I can't much more without giving stuff away. It is a satisfactory tale with several twists I did see coming.