NotebookLM Enterprise (part of Gemini Enterprise on Google Cloud) has a preview API on the Discovery Engine service: https://<location>-discoveryengine.googleapis.com/v1alpha. The consumer NotebookLM has no API.
Use a Google Cloud OAuth access token for an account with NotebookLM Enterprise access — for testing, gcloud auth print-access-token. Send it as Authorization: Bearer <token>. In the examples it's written {{GCLOUD_TOKEN}}; {{PROJECT_NUMBER}} is your project number.
Full reference: https://docs.cloud.google.com/gemini/enterprise/notebooklm-enterprise/docs/api-notebooks
curl -X POST https://us-discoveryengine.googleapis.com/v1alpha/projects/{{PROJECT_NUMBER}}/locations/us/notebooks \
-H "Authorization: Bearer {{GCLOUD_TOKEN}}" \
-H "Content-Type: application/json" \
-d '{"title": "PostTaco research"}'curl https://us-discoveryengine.googleapis.com/v1alpha/projects/{{PROJECT_NUMBER}}/locations/us/notebooks:listRecentlyViewed \
-H "Authorization: Bearer {{GCLOUD_TOKEN}}"