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Is there enough interest to warrant the time and effort to write the book?

You could also ask if there's any point in making music software. The market is saturated, and musicians aren't making any money, for the most part.

But, all those people who have music making software and who aren't making money are looking for a niche to explore where they might actually make the software pay for itself. Jingle writing is such a niche.

So it becomes a moot point whether or not there is a need. There is a POSSIBILITY, and that seems to be enough for most people to make a purchase.

I say go for it, if you have the time and knowledge to undertake such a project.