The papers to read to get started:
A discrete history of the Lorentzian path integral
Reconstructing the Universe
Then the usual:
Dynamically Triangulating Lorentzian Quantum Gravity
Emergence of a 4D World from Causal Quantum Gravity
Semiclassical Universe from First Principles
Spectral dimension of the Universe
(Thanks to this post from Marcus)
And finally, the current literature on ArXiv.