While most people interested in a Surface Book have probably already gotten one, there's some bad news for people who haven't yet: The terabyte version of the Surface Book has been dropped from the online Microsoft Store. Without any kind of announcement or other fanfare from Microsoft, the option to get the most high-end version of the laptop has disappeared from the online marketplace.
— Microsoft Surface (@surface) July 6, 2017
There are rumors that the Surface Book may be getting the next device in its line sometime soon, but for right now, we can only interpret this as the current Surface Book getting production toned down. If you truly need the 1tb version of the Surface Book, you probably won't have a problem picking one up using some kind of third party service, like Amazon - or even a retailer.