In the end, there will be 81 short novels in the series. You can find the official list and their order on the Current Books List page.
To learn more about the series, check out the ghosts' website. http://ghostsofcullodenmoor.weebly.com Yes, they have their own, though we are woefully behind on getting all their information added. In the end, it should be quite impressive, but for now, you could learn something you may not have read in the books.