・。
He's an enthusiastic reader of Stephen King.
Literal
He [topic-は] Stephen King [genitive-の] enthusiastic devoted-reader is.
A short book-lover description. 愛読者 (あいどくしゃ) is a compound: 愛 (love) + 読 (read) + 者 (person) = 'a devoted reader,' 'an ardent reader' — the Japanese counterpart of 'fan' or 'aficionado' specifically for reading. The collocation 熱心な愛読者 ('enthusiastic devoted-reader') doubles up on the enthusiasm. Note that 愛読者 inherently implies enthusiasm, so 熱心な is in some sense redundant — but the combination is a standard intensifier in book-review and literary writing. Classic example of how Japanese builds compound nouns with a 者 ('person') suffix for 'person-who-does-X' labels.