This is the book's page at MIT Press: https://mitpress.mit.edu/books/bicycling-science-fourth-edition. Direct ordering is no longer available but there are several options:
Indiebound.org https://www.indiebound.org/book/9780262538404
https://www.powells.com/book/bicycling-science-9780262538404
https://www.waterstones.com/book/bicycling-science/david-gordon-wilson/theodor-schmidt/9780262538404
https://www.chapters.indigo.ca/en-ca/books/bicycling-science/9780262538404-item.html?ref=isbn-search