The functions are documented here. Keys are not documented, but hinted (its not possible to document them all as they are not all owned by Adobe). The preferences have to live somewhere and you can take a look at the preferences file to find out what preferences have been stored along with their names by opening the prefs file in a text editor.
You can find the the preference file is located in:
*%appdata%\Adobe\Adobe Illustrator x.x Settings\en_GB\AIPrefs%appdata%\Adobe\Adobe Illustrator x.x Settings\en_GB\AIPrefs on windows computers. Where x.x is version, and language folder may differ.
*Users//Library/Preferences/Illustrator x Settings/en_GB/AIPrefUsers//Library/Preferences/Illustrator x Settings/en_GB/AIPref on OSX. Where X is version number and language folder may differ.
A relevant excerpt from this file file looks like follows:
/maxUndoDepth 200
/undoDepth 5
/Grid {
/Horizontal {
/Spacing 34.0157470703
/Ticks 2
}
/Vertical {
/Spacing 34.0157470703
/Ticks 2
}
/Posn 1
/Style 0
}
This looks remarkably like PostScript or PDF dictionaries. So these are then the keys denoted with a slash (/) in front and sub dictionaries or arrays delimited by curly braces ({,}). To access a sub key you just add keys together with /.
So getting prefs for undo depth should look as follows:
app.preferences.getIntegerPreference('undoDepth');
While getting a subkey like spacing looks like:
app.preferences.getRealPreference('Grid/Horizontal/Spacing')
Note take care with asking keys, as a key is created every time you ask if it does not exist. The names are case sensitive.
* anybody who knows where its on mac please let mne know or edit it in