소개자가 좋은 책이라고 해요. "The introducer says it is a good book." ☆좋은 책이라고☆ 좋은 - good (determiner) 책 - book 이 - is (이다) 라고 - (informal) plain imperative indirect quote [that ... should/will] (technically: 라 - informal imperative morpheme 고 - indirect quote morpheme) [that] it will be a good book -> ☆it is [going to be] a good book☆
소개자가 - The introducer (or, equally reasonable sounding in English, The cow-dog-person :P) 해요 - says (takes on this meaning in -고 indirect quote constructions) ★The introducer says ☆it is a good book☆★ ★소개자가 ☆좋은 책이라고☆ 해요.★
Recommender is really just awkward in English and shouldn't be considered correct. I went with "introducer," not because it's better but because it's equally awkward and had no idea what was expected. It works in Korean but in English it would have to be "the person who recommended the book says it's good." Honestly, I think this lesson needs a rewrite. There are too many awkward sentences that are probably tripping up a lot of learners.