。
She's bored with this novel.
Literal
She [topic-は] this novel [with-に] is-bored.
退屈する ('to be bored') takes に for the source of the boredom — Japanese consistently uses に for the object of mental and emotional states (with 慣れる 'get used to,' 興味がある 'be interested in,' 同意する 'agree with'). The polite present continuous ~ています captures her current state of boredom rather than a past episode. 小説 is the standard word for 'novel,' a Sino-Japanese compound (小 'small' + 説 'theory/account') — the etymology reflects how novels were originally seen as minor literary forms compared to serious essays and poetry, before becoming the dominant prose genre.