。
She is holding a book under her arm.
Literal
She [topic-は] book [object-を] one-volume under-arm [at-に] is-holding.
小脇にかかえる ('to hold under one's arm') is a vivid descriptive phrase — 小脇 literally means 'small side.' The counter 一冊 (いっさつ) is the standard counter for books and bound volumes.