Q. If I found a book that I thought would make a great movie, and I bought the rights, on what basis could I then hawk it around? Am I still a spec writer? Or am I a production company looking for partners/backers?
Realistically if you're looking to write it, you're a writer, and if you're looking to set it up, you're producing it.
As a writer you'd be expected to have a "take" on how you'd adapt the book: basically pitch the story as you see it on screen. As a producer you'd want to write up a short and snappy pitch on why the book is a great property to adapt, but you wouldn't have to explain how you'd adapt it.
Only a successful book really adds commercial value to the project; only a bestseller is really a bankable element. If no one's heard of the book, then it might actually be a strike against the project: the book already flopped, who is gonna want to see a movie about it?
(In Canada, though, the mere fact of a book being Canadian is a plus when you go to Telefilm for development money.)
If the book isn't a hit, then ask yourself how closely you're going to adapt it. If the characters and plot are very clever indeed, then you'll need to option the book. If what you really like is the concept, then you might be better off considering the book "inspiration." No one can copyright a concept. So long as you don't use the original characters or plot, you are free to steal the book's concept -- that is, until you sign an option agreement with the author. Then the book is attached permanently to the project.