Where do I find documentation on Eagle XML file structure?
The Eagle installation includes a DTD file that describes the format, from the Eagle V6 documentation:


  The internal data format is based on XML. The data structure is documented in the eagle.dtd which can be found in the doc folder of your EAGLE installation.


On my x64 machine this was located at C:\Program Files (x86)\EAGLE-6.4.0\doc\eagle.dtd. The header of the file sums it up and hopefully this covers what you're after:


  EAGLE version 6.4.0 DTD Copyright (c) 2013 CadSoft Computer GmbH   http://www.cadsoft.de This file describes the XML file format used by EAGLE version 6.4.0, hereafter referred to as the "EAGLE File Format". It is made available under the creative commons "CC BY-ND 3.0" license (see http://creativecommons.org/licenses/by-nd/3.0). You may use this file to implement a program that reads and/or writes files in the EAGLE File Format. If your program writes files in the EAGLE File  Format, these files must be readable by EAGLE version 6.4.0 without any error messages or warnings.