There is a newish #openaccess textbook on The Physics Of Interstellar Travel by Coryn A. L. Bailer-Jones.
Textbook: https://www.cambridge.org/core/books/physics-of-interstellar-travel/69C23CDCF164E2C71A65497F299EE38F
Author's Textbook Webpage: https://bailer-jones.www3.mpia.de/pit.html
Author's Youtube channel with chapter-by-chapter summary videos: https://www.youtube.com/@physics-of-interstellar-travel
