In my world there are different races and creatures. I'm currently writing one book in the series, but in this book I have no need for any of the characters (other than background people) to be of different races. I also have no need of any "other world" creatures. The main characters all humans, and that's how I picture them to be. Would it be odd for this one book to have all humans as main characters (or those important to the story goal), and then have different races as main characters in the other books?

I don't think it would be odd, especially if the story gives a reason for why humans are the main characters in that part of the story, and it's not portrayed as a coincidence.

It might be odd. Or it might not. It depends on the world (if there are these other races and creatures, but they're rare, it's not unreasonable for the characters of one book to never run into them) and the circumstances. Basically, whether you give it in the course of a book or not, you should have an answer (and a "Watsonian" rather than "Doylist" answer, i.e. an answer that makes sense within the world of your books rather than one based on what you needed to tell a particular story) for the question "... so if there are these fantastical creatures and non-human races, why didn't these characters ever run across any?"

I don't think it would be odd, personally; as long as you have the other races in your first book, so they're not appearing in your second book out of nowhere, then I don't see that there would be a problem with it.