。
She gave me a book as a Christmas present.
Literal
She [topic-は] Christmas-present [as-に] book [object-を] gave-to-me.
Three layers worth noticing. クリスマスプレゼントに uses the に of role or capacity — 'as a Christmas present, [she gave a book].' This is the same に as in 結婚祝いに ('as a wedding gift') or 記念に ('as a keepsake'). くれる is the giving verb specifically for gifts directed toward the speaker or the speaker's in-group — distinct from あげる (give to outside) and もらう (receive). Choosing くれる here makes the recipient implicit — the speaker — without needing 私に. In Japan, Christmas evolved into largely a romantic and gift-exchange holiday rather than a religious one, with December 24th (the eve) being the bigger date for couples and the cake-and-strawberry tradition unique to Japan.