Buy a used paperback copy on Amazon or AbeBooks for $30. Spend 20 minutes at a university library scanner. Most library scanners email you a searchable PDF for free. You now own the legal physical copy and a digital backup.
Here are a few suggestions:
Wen Shen’s approach emphasizes the balance between mathematical rigor and practical implementation, ensuring that readers understand not just how to code an algorithm, but why it works. Key Pillars of Wen Shen’s Introduction an introduction to numerical computation wen shen pdf