She was absorbed in comic books.

Literal

She [topic-は] comic-books [to-に] was-absorbed.

漫画本に夢中になっていた — 夢中になる (むちゅうになる, 'to become absorbed/engrossed') is a set expression. に marks what one is absorbed in. 漫画本 (まんがぼん, 'comic book/manga volume').