While by Barton Zwiebach is a copyrighted textbook published by MIT Press and is not legally available for free download in its entirety, there are several legitimate ways to access its foundational material and related resources at no cost. Legal Ways to Access Zwiebach's Quantum Mechanics Content

: MIT provides official resources such as an errata PDF and sometimes sample chapters through platforms like Google Books . Where to Purchase the Full Textbook

If you are looking for the content contained in "Mastering Quantum Mechanics," you can find the original lecture notes and course materials that formed the basis of the book through and other academic platforms.

: This book is based on the lectures and extensive notes Barton Zwiebach developed for his undergraduate courses at MIT. You can find comprehensive lecture notes for Physics 8.04 and Physics 8.05 available for free online.

: The textbook is directly tied to the popular MITx "Mastering Quantum Mechanics" series. While obtaining a certificate requires payment, you can often "audit" the courses on MITx Online for free to access the instructional videos and some materials.

For those who want the complete 1,000+ page comprehensive volume with all updated exercises and theoretical foundations, the book is available from several authorized retailers: Quantum Physics I - MIT OpenCourseWare