From reading both this blog and queryshark, I know being a debut author with no prior publication credits as far as novels go help your odds when submitting to agents.
I will soon have the opportunity to collaborate with a more established author on a setting book for a tabletop roleplaying game.
If this is published and my name is listed as one of the authors, does this mean I will no longer be considered a debut author to prospective agents?
Or is being a contributor for an rpg book more like writing a short story for an anthology? To my knowledge, publication credits of short stories for literary magazines and such are an asset when it comes to submitting to agents.
If doing this would harm later chances at publication of my debut novel, do you think I should defer this chance to collaborate until some other time? If it will help my odds, is there any reason not to do it?
First rule of writing: don't turn down paying gigs!
But to answer your question: you're in no danger here of not being a debut novelist. IF some sort of contest requires you to have published nothing previously (ie not just no previous novels) well, you'll have to pass that one by, but "debut" is mostly a tool for marketing and publicity.
Your novel will be your debut novel.
Much like your eldest daughter is your first daughter even if she has an older brother.
Here's what you need to remember: the reason everyone is on the hunt for debut novels is not cause they're debuts, it's cause we won't have to explain why your last novel didn't sell well. Career resuscitation is not for the faint of heart, agent or author.
Thus, sales for a book for the roleplaying game industry with a specific target audience won't have much bearing on whether an indie bookstore in Dubuque, or the crime list buyer at B&N will want to stock your novel.