What is the best reference book to help you learn ancient Chinese characters?

Answer in two steps, as follows.

Introduction of auxiliary materials for learning ancient Chinese;

Compared with vernacular Chinese, classical Chinese is nothing more than awkward and obscure. If you want to devote yourself to learning ancient Chinese, you can't limit your horizons.

In the Chinese book. Recommended supplementary materials: Introduction to China's ancient literature, Zi Tongzhi Jian, Historical Records, Analects of Confucius, Mencius, Han Feizi, etc.

Introduction of auxiliary materials for learning ancient Chinese characters;

The term "ancient Chinese characters" is incorrect, but it should be called "Chinese characters". The reform after 1952 is called "simplified Chinese characters". Recommended auxiliary materials: Copybook for calligraphy, Traditional Chinese Dictionary, Taiwan Province News Today website, Simplified and Complicated Characters, etc.