Update February 2018: The download link, below, is now to new data fresh from CoE 3.26!

Like before, the format is c3m files (plain text files) with pseudo-mod code. And, for the best reading experience, I recommend a text editor with CoE syntax highlighting, such as the schemes I have made for Gedit and Notepad++ – both of them incidentally now updated for v3.26 – or, if nothing else, the syntax highlighting of just a Unix shell script. (The last one will at least make comments and string literals stand out from the rest.)

Whatever you do, I would advise you don't use the Notepad that comes with Windows. (If you're on Windows, you'll be much better off with Notepad++[notepad-plus-plus.org].) Microsoft Notepad and CoE modding (or just about any serious text processing) are two things that simply don't belong in the same sentence.


The rest of this post is what I originally wrote in May 2016:
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So...

I finally got around to take a good look at what the class and recruitment data in the executable actually look like, and found they weren’t at all as horribly indecipherable as I had feared. In particular, data seem to be generally stored in fixed-size blocks, thankfully also often begun with some telltale plain text string. Which greatly facilitates comparison between blocks, in order to figure out what each byte and bit might represent. Numbers seem to be mostly 4-byte (signed) integers, each beginning at an offset evenly divisible by 4. (When using a hex editor, I find it helpful to have it display bytes in groups of 4.) Although there are a few 2-byte integers too.

The ideal, in my view, would have been human-readable text files (e.g. like mod files in format) with the definitions of all the stuff in the game, classes, units etc. In their absence, though, I believe things could have been worse than they are.
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I was eventually even able to put together some Python scripts to extract the data into...

• text files with pseudo-mod code for download[www.dropbox.com].

Not meant to be used as actual mods, of course. (There aren’t mod commands for all the data in the executable, and I don’t understand all of it anyway; see the comments.)

I could now kick myself for not doing this earlier. I did in the past actually waste some considerable time trying to establish only some of this through arduous tests.

For the best reading experience, I would recommend a text editor with CoE syntax highlighting, such as the schemes I have made for Gedit and for Notepad++, or if nothing else the syntax highlighting of just a Unix shell script (will at least make comments and string literals stand out from the rest).
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Notes:

• The file on standard recruitments (standard troops, standard wizards, paid heroes and assassins) is not extracted from the executable, as I cannot there identify the data for it. It contains guesstimates based mostly just on tests that I also now got around to perform. I now feel fairly confident this is how it really works, though there is almost no way to be entirely sure.
  
  (Again, I wish I had done this earlier. There have been times playing this game when the capturing of extra libraries felt almost like a pointless ritual based purely on superstition, as I had so little an idea what the actual probability bonuses were.)
  
  
• The other files contain data actually found in the executable – complete with any errors there might be. Not that I expect there to be many errors, of course, but user Ridiculus recently discovered what seems to be a corruption of recruitment data for the witch (which must have happened in a patch).

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For the curious, interested in a comparison with CoE4, I posted the corresponding CoE4 data in the CoE4 forum.