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.
Finite Element Mesh Generation (Daniel S.H. Lo), CRC, 2015. This book also covers several topics related to mesh generation for finite elements. The style is less mathematical-oriented in comparison with the previous one. The book has code for several algorithms in C++ or Fortran, but they could be better formatted.
Delaunay Mesh Generation (Cheng, Dey, Shewchuk), CRC, 2012. Covers some of the really challenging implementation details for Delaunay meshing in 2D and 3D, like exactly how all the edge and face flips work, sliver exudation, etc.
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.
Joseph O'Rourke. Computational Geometry in C. A book that gives overview of different geometry algorithms such as point in polygon.