I find it very hard to find books on Lorentzian Geometry, more focused on the geometry behind it, instead of books that go for the physics and General Relativity approach. More specifically, I'm talking about the Lorentzian manifolds and Lorentz-Minkowski spaces (some notations of it are $\Bbb L^n$, $\Bbb E^n_1$, etc). I know that the subject is recent (about $15$ years or so?), so we might not have a lot of texts about it anyway, but it costs nothing to try.
I am not talking about isolated papers and articles, but of texts which make a systematic approach of the subject.
I thought of making a list here, so we can gather some material, the most we can.
I'll make a CW answer, and I invite everyone who knows something about it to give their two cents.
Thanks.
List of books and texts on Lorentzian Geometry:
Differential Geometry of Curves and Surfaces in Lorentz-Minkowski space (also 1st arXiv version) - Rafael López;
Semi-Riemannian Geometry With Applications to Relativity - Barret O'Neill; (does a bit about the specific subject asked)
An Introduction to Lorentz Surfaces - Tilla Weinstein;
Curves and Surfaces in Minkowski Space - Johan Walrave; (PhD thesis, hard to find)
Linear Algebra and Geometry - Igor R. Shafarevich, Alexei O. Remizov; (from pages $265$ to $288$)
An Introduction to Lorentzian Geometry and its Applications - Miguel Javaloyes, Miguel Sánches.