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She saw that there were several books on top of the piano.
Literal
She [topic-は] piano [of-の] top [on-に] book [subject-が] some-volumes exist [nominalizer-の] [object-を] saw.
何冊か wraps a question word (何) with the counter for bound volumes (冊) and tags か at the end to mean 'some, a few' — a fixed pattern: 何人か ('a few people'), 何回か ('a few times'), 何個か ('a few items'). The whole proposition '本が何冊かある' ('there are some books') is then nominalized with の and treated as the perceived object via を — 'she saw [the fact that] there were some books on the piano.' This 'verb of perception + のを' pattern (見る, 聞く, 感じる) is the standard way to embed a propositional perception.