Interesting. But there are so many (marketing) things wrong here, I don't know where to start. For starters...
1) The Offer - Free might have been the wrong approach. If there's an audience that's familiar with "you get what you pay for" it's engineers. That is, free is a signal of perceived value. Pricing it at (e.g.) $15 and offering (e.g.) three free chapters could have fared better.
2) The Offer: Part 2 - Price and size of free sample should have been tested. Perhaps $25 and four free chapters would have done better? Or $20 and two free chapters.
3) Testing - It sounds like he went all in on all the distribution channels immediately. Instead, pick one or two and *again* test some ideas and offers and don't scale until you have some confidence in the banner, text, verbiage, offer, etc. whatever.
3.5) For that matter he should have done some testing of what he picked. For all we know, X (fka Twitter) might have been better. He assumed that what he reads is best for sales. It might reach the target but that doesn't mean anything. *No one* is smarter than The Market. And the only way to understand The Market is to test.
4) (Lack of) Repetition - I didn'n't see an exact number of weeks - maybe I missed it? - but getting someone's attention to the point that they act takes time. It takes that person seeing the ad over and over and over.
5) (The Wrong) Assumption - The smartest theory to start with is "No one cares. No one cares about this product the way we do. We have to make them care."
Put another way, your baby isn't just ugly, it's invisible. Deal with it.
Etc. Etc. Etc.
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Interesting stuff, but in many ways this is a text book case that could have been titled: Marketing & Sales - How Those Who Write Code Get It All Wrong.
With that title / scope he could write a second book and sell more than the first one.
