。
She writes essays as well as novels and poetry.
Literal
She [topic-は] novels [and-や] poetry [in addition to-に加えて] essays [also-も] writes.
This sentence layers three additive constructions. に加えて is a slightly formal way to say 'in addition to,' more elevated than のほかに. The particle や lists items non-exhaustively — implying she may write in other genres too — while も adds the emphatic 'also' to エッセイ, reinforcing that essays are yet another thing on top of the rest. The plain form 書く at the end expresses habitual action.