If a book is genuinely out of print and no new copies are being sold, some jurisdictions allow "abandoned works" provisions. However, that is a legal gray area. The safest route is always legitimate purchase or library borrowing.
However, Das’s book endures because it offers something AI struggles with: curated, sequential, error-checked problem sets . A PDF is merely a container. The value lies in Das’s pedagogical sequence. Mechanical Vibrations Jbk Das Pdf