What you are seeing is flow — the state where your child feels their best and is fully engaged. Your teen is absorbed in a book and the required work is waiting. At this age, the management of this tension is almost entirely theirs to own. A teen who reads deeply for pleasure is doing something neurologically and academically valuable. A teen who can also transition to required work without external management is demonstrating genuine self-regulation. Your role here is not to interrupt. It is to hold the expectation clearly and trust them to figure out how to meet it.
I trust you to manage your time. The work needs to be done by tonight. How are you planning to handle it?