Probably should also mention the Art Defines XML files which are inside the FPK files:
civ5artdefines_citybuildings.xml
civ5artdefines_eventeffects.xml
civ5artdefines_improvements.xml
civ5artdefines_landmarks.xml
civ5artdefines_landmarktypes.xml
civ5artdefines_overlay.xml
civ5artdefines_sv_features.xml
civ5artdefines_sv_improvements.xml
civ5artdefines_sv_resources.xml
civ5artdefines_sv_terrain.xml
civ5artdefines_sv_units.xml (*)
civ5artdefines_unitmembers.xml (*)
civ5artdefines_unitmembertemplates.xml
civ5artdefines_units.xml (*)
civ5artdefines_viseffectmodels.xml
civ5artdefines_viseffectparticles.xml
civ5artdefines_viseffects.xml
civ5artdefines_wonders.xml
* - Dealt with in
Kael's modding guide
Some of these have XSD schema definitions:
civ5artdefineschemas_unitmembers.xsd
civ5artdefineschemas_units.xsd
civ5artdefineschemas_wonders.xsd
civ5artdefines_viseffects.xsd
For added consistency, the fog color and terrain textures seem to be defined by these CSV files:
fogcolor.csv
terraindef.csv