The bookstore is undergoing a period of transition and will return at some point soon maybe or maybe not idk. I am planning to move away from having an integrated bookstore on the site using the Amazon API, since it wasn't really working and probably seemed sort of sketchy anyway. I am replacing in-content links to bookstore items with links to the appropriate material on as I come across it, and it is possible I will not have any sort of dedicated bookstore page at all in the future, I'm not sure.