It's easy to miss the video on the front page, which I find provides a great visual summary of features and will make you understand why other commenters are praising how efficient (and pleasurable, I might add!) TeXmacs is: https://www.texmacs.org/tmweb/home/videos.en.html.
The best in-depth reference, even counting the astoundingly complete bundled manual, remains The Jolly Writer. It is a beautifully typeset book, available at https://www.scypress.com/book_download.html.
You can find some example documents here https://texmacs.github.io/notes/docs/example-documents.html.
Other posts on the TeXmacs notes site discuss programmability with Scheme, typesetting math (https://texmacs.github.io/notes/docs/texmacs-math-typesettin..., shows how good the HTML export is), and more.
The best in-depth reference, even counting the astoundingly complete bundled manual, remains The Jolly Writer. It is a beautifully typeset book, available at https://www.scypress.com/book_download.html.
EDIT: missing link, typo