Yes, most of the Tsubook app is accessible without Internet.
Internet is mostly important at the moment you get Tsubook. First, you need to be connected to Internet to download Tsubook. After that, the first time you open Tsubook, you will need Internet to create (or connect to) an account.
Once connected to your Tsubook account, you can use the app offline to check all the knowledge (points, meridians, categories, indications,…).
Internet will be useful for everything related to your Tsubook account (connection, license management,…) and some specific actions (referral program, partnerships,…).