、。、。
The girl was holding a few books in her arms. Not textbooks — thick, hardcover books.
Literal
That girl [topic-は], several volumes [gen-の] books [obj-を] was-holding. Textbook is-not-[and-で], thick hard-cover.
Narrative prose describing an observation. 数冊 uses the 冊 counter with 数 ('several') — a compact way to say 'some books.' 抱える ('to hold in one's arms') is a specific holding verb distinguishing armful-carrying from 持つ (generic hold). 上製本 ('hardcover book') is a Sino-Japanese publishing term. Note the second sentence's minimal structure: '[negation], [positive description],' fully nominal. This is a characteristic Japanese narrative device where the narrator elaborates without repeating the verb.