She [topic-は] bookshelf [at-に] exist several books [object-を] him [to-に] showed.
本棚にある数冊の本を彼に見せた — a relative clause (本棚にある, 'that are on the bookshelf') modifies 本. 数冊 (すうさつ, 'several volumes') uses the book counter 冊. 見せる (みせる, 'to show') takes に for the person shown to.