One consideration, though not the only one, is that writing a technical book is never a good financial proposition.
If it's not a good financial proposition already, why write a book at all, vs making a website or a series of blog posts? The answer is usually prestige.
So, in general, I would say don't write a book unless you are doing it for your resume, otherwise there are better formats for technical content.
For your specifics, I wonder if the real benefit was you helping him through the material, rather than the method you used. We continue to have teachers despite there being tonnes of books on most topics people want to learn.
