In short, his new novel can be said to be a disappointing, boring work.
Literal
Essentially, his [possessive-の] new novel [topic-は] expectation-[failing-はずれ] [attributive-の] boring work [quotative-と] can-say.
期待はずれ ('disappointing,' literally 'expectation off/missed') is a compound noun 期待 ('expectation') + はずれ (from 外れる 'to miss/come off'). ~と言える ('can be said to be') is a critic/review register. 作品 ('work') specifically means a creative/literary/artistic piece.