。
She wrote a book about animals.
Literal
She [topic-は] animals [concerning-に関する] book [object-を] wrote.
に関する (concerning/regarding) is a compound particle that modifies a noun — here linking 動物 to 本. It's more formal than の (動物の本 would mean the same thing casually). This pattern is characteristic of written Japanese, academic descriptions, and professional contexts.