Open the PDF to Chapter 3 (Block Ciphers). Skip the introduction. Read the section on the Feistel Cipher structure three times. Then, try to draw it from memory. That is when the magic happens.
Insights into SSL/TLS protocols that secure every transaction on the modern web. Cryptography And Network Security By Atul Kahate.pdf
Here is a breakdown of why the content is considered "solid" and what makes it a standout resource in the field. Open the PDF to Chapter 3 (Block Ciphers)