The I Ching (Book of Changes) is the first of the Chinese classics. Many think it's a divination book — but it's actually a philosophy of change.
Taiji gives birth to two forms: Yin (broken line) and Yang (solid line). Everything has two sides: heaven/earth, male/female, day/night. But Yin contains Yang and Yang contains Yin — that's the Taiji diagram.
Three lines form a trigram: Qian ☰ (heaven), Dui ☱ (lake), Li ☲ (fire), Zhen ☳ (thunder), Xun ☴ (wind), Kan ☵ (water), Gen ☶ (mountain), Kun ☷ (earth).
Two trigrams stacked form a hexagram. Qian over Qian = "Creative Heaven." Kun over Kun = "Receptive Earth." Each has judgment texts and line texts describing a state and its changes.
Don't start with divination. Read the Ten Wings (commentaries) first to understand the philosophy. Then read the Sequence of Hexagrams to see the logical progression.