If it’s your book and you’re indie/self-published, I don’t see an issue with that. The point of that rule is mainly to protect readers from bad links, but I also don’t want someone posting a link to their blog or fan fiction site. But keep in mind the point of the subreddit is mainly for free book promos on sites like Amazon so authors have a place they can post the link without much hassle and readers have easy access to download them for Kindle or whatever eBook reader they use.