Starting a community answer, in line with your model question

Mesh Generation

Mesh Generation: Application to Finite Elements (P.-L. George and P. Frey) Hermes, Lyon, 2000. A clear primer on the core technology and terminology of mesh generation. Written in a fairly mathematical style, which might not appeal to those of a more practical outlook. Will fit well with Gmsh, due to the cross-pollination of authors and project.

Geometry and topology

Edelsbrunner, Herbert. Geometry and topology for mesh generation. Vol. 7. Cambridge University Press, 2001. This book concentrates on creation of meshes of triangles and tetrahedra, and the mathematics behind them.