Are you willing to spring for a pro editor to go over the book so it can be a shinning star in your cap?

I don't think you need to start out with a this is what I am ging to do book. Non-fiction and Fiction are different enough--but that's just my opinion.

Vanity publishing is when you pay a subsidy publisher to publish your book (and you pay all the other costs relevant to it, including stuff that goes for subsidy publishing people's salaries).

If you self-publish, you go to the printer, make the arrangements, do the "publishing" work yourself, and save lots of money. (You still have to pay, but you're only paying for the printing and paper and so on, you're not paying someone else's salary.)

Anyway, that isn't your real question, but I did want to clarify that. Self-publishing is far, FAR better than subsidy ("vanity")publishing.

Now, to your real question.

It can be argued that any writing will give you writing practice. Also, the very fact that you have completed a book for a real organization can be considered a plus. And even though it's not published by a big-name publisher, it would still count as a credit.

It tells publishers that you can handle big projects, that you can put sentences together and have them make sense, that you can work with people who have their own ideas about how the task should be done and work with them professionally.

Such a credit would show that you have had some experience, even if it isn't experience in the field you really want to write in.

It seems to me that the real question here is whether or not you want to do it. If you want to do it, it certainly won't hurt your career plans (except, perhaps, in delaying them while you complete this project).

I'd go for it, my friend. It's volunteer work, and what's more, they are asking you. You are not asking them. I think that would make a great impression on the minds of future publishing houses.

It would show, as K says, you can handle a large project, you can do the work, and you've been the the whole process at least once.

As for your question: Should it come from my creative mind or be work for someone else. Most writers get a degree in something else, a job they don't love, until they make it. Lots of writers become freelance writers for mags and reporters, and journalist, and spend years writing stuff they don't love. I really don't even think that's an issue. Hey, even Card's first book was some sort of pychological text book about parenting.

I think it will only help you, but, don't do it unless you want to.