Do you have a link to the book you are talking about?
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Automation scripts:
Assets\Automation\AutomationStartup.lua
Assets\Automation\DailyPerfRun.lua
Assets\Automation\DailyPlaytestRun.lua
which looks like it would be useful to hijack to create "autoplay" options.
Here are the Lua gameplay files:
Assets\Gameplay\Lua\AssignStartingPlots.lua
Assets\Gameplay\Lua\ContinentsWorld.lua
Assets\Gameplay\Lua\FeatureGenerator.lua
Assets\Gameplay\Lua\FLuaVector.lua
Assets\Gameplay\Lua\FourContinentsWorld.lua
Assets\Gameplay\Lua\FractalWorld.lua
Assets\Gameplay\Lua\GameplayUtilities.lua
Assets\Gameplay\Lua\HintedWorld.lua
Assets\Gameplay\Lua\MapGenerator.lua
Assets\Gameplay\Lua\MapmakerUtilities.lua
Assets\Gameplay\Lua\ModBrowserUtilities.lua
Assets\Gameplay\Lua\MultilayeredFractal.lua
Assets\Gameplay\Lua\PangaeaWorld.lua
Assets\Gameplay\Lua\TerrainGenerator.lua
Assets\Gameplay\Lua\WorldBuilderRandomItems.lua
that looks like the equivalent of the python world building files.
There also appear to be lua files in the map directory:
Assets\Maps\Four_Corners.lua
Assets\Maps\Fractal.lua
Assets\Maps\Great_Plains.lua
Assets\Maps\Highlands.lua
Assets\Maps\Ice_Age.lua
Assets\Maps\InlandSea.lua
Assets\Maps\Lakes.lua
Assets\Maps\North_vs_South.lua
Assets\Maps\Oval.lua
Assets\Maps\Pangaea.lua
Assets\Maps\SmallContinents.lua
Assets\Maps\Terra.lua
Assets\Maps\TinyIslands.lua
Assets\Maps\West_vs_East.lua
which implies that the Gameplay lua files are libraries rather than kinds of map.
Here is a terrain graphics piece:
Assets\terrain\pieces\Desert Hill\Africa\Afri_Desert_Hill_1_1.csv
Assets\terrain\pieces\Desert Hill\Africa\Afri_Desert_Hill_1_1_H.dds
Assets\terrain\pieces\Desert Hill\Africa\Afri_Desert_Hill_1_1_H_BLEND.dds
Assets\terrain\pieces\Desert Hill\Africa\Afri_Desert_Hill_1_1_T1.dds
Assets\terrain\pieces\Desert Hill\Africa\Afri_Desert_Hill_1_1_T_BLEND.dds
4 dds files and a csv. (direct draw surface and a comma seperated value files)
Here is the list of cursors (from which we can derive commands with targets):
The Assets\UI lua files are pretty comprehensive. They contain lua files for most start-up menus in the game to start with -- was that true in civ4?
Plus .lua files for most in-game dialogs. Is that how final frontier worked?
There are a bunch of gr2 files that are overlays, for things like flanking.
The tutorial is implemented as a bunch of map and lua files:
which sounds good, from a scenerio modding perspective.
I don't see strong evidence of a "split units into their own XML files for ease of modular modding". Hopefully some of the modular XML stuff migrated from civ4 to civ5.