。
She was absorbed in reading.
Literal
She [topic-は] reading [in-に] became-absorbed.
ふける ('be absorbed, be lost in') is a slightly literary verb of immersion. に marks the activity into which one is immersed. Common pairings: 物思いにふける ('be lost in thought'), 思い出にふける ('be lost in memories'), 読書にふける ('be absorbed in reading').