Instead of buying books individually, SAP Press offers a subscription service.
Use SAP Help Portal (help.sap.com). This is the official product documentation. It is dry, but it is 100% accurate and free. Combine it with a borrowed SAP Press book to understand the "why" behind the documentation. Download Sap Press Books Free
Scribd (rebranded as Everand) offers a monthly subscription for $11.99. They have a surprising number of SAP Press books from 2017-2020. Not the latest editions, but excellent for foundational learning. Cancel within 30 days for a full refund. Instead of buying books individually, SAP Press offers