。
She expressed her thanks for his help in her book.
Literal
She [topic-は] her own book's [inside-の中で] his help [for-に] gratitude [object-を] expressed.
謝意を述べる ('to express one's thanks/gratitude') is a formal collocation. 述べる ('to state, articulate, deliver') is the standard verb for stating something formally — typical of speeches, written acknowledgments, and official correspondence. 謝意 ('gratitude, thanks') belongs to the same elevated register; the everyday spoken equivalent is more often 感謝 or 感謝の気持ち. The phrasing fits a printed acknowledgments page.