When you create a Leanpub book or course, you get to choose a URL. Of course, if someone else is already using the URL you wan't it won't be available.
We have also restricted some URLs from being chosen, such as common words or abbreviations like "tbd". If you try to use a restricted URL, you will not be able to create your book.
We recommend authors choose short and unique URLs. For some tips, please see this article: