I know that it's easier to get signed as a debut novelist than after having published a book, especially if the book didn't sell very well. (Your exact words were, "It's easier to launch a career than to revitalize one.") So my question is, what if the first book wasn't a novel?
I'm one of three writers at a niche blog, and we'd like to create a nonfiction advice book. The blog is currently hosted by a small publishing company, which is a nice connection, though of course there are no guarantees! Because we have a narrow audience, it's unlikely that we'd sell an impressive number of copies.
But I'm a woodland creature! My question is, how will this affect my future chances of publishing a novel? Publishing credits are good, but then I wouldn't be a debut author, even though it would be my first novel.
But you will be a debut novelist, and that's what counts.
Non-fiction has very little crossover to fiction in terms of sales. Just ask Bill O'Reilly who has had several "non"-fiction books (at least that's how they are marketed--historians are too busy laughing to explain why it's hilarious) on the top of the best-seller lists, but his novel has tanked not once but twice.
Think of it this way: your non-fiction advice book about How To Dry Clean Your Dragon isn't going to appeal to the same audience that wants to read your novel about ballerinas in space.
People read advice books to learn things.
People read novels to be entertained.
You'll be just fine.