Publishing houses, self-published writers, and everyone who publishes need proofreaders. There's fiction and nonfiction.
Good copyeditors emphasise style, formatting, clarity, and flow in addition to grammar. Fact-check or suggest improvements to improve the writing.
Communicating in various languages is a highly sought-after ability. Translating written works is a good side job for a bilingual reader.
If you speak well and have a compelling voice, narrate audiobooks. You'll get paid to read books aloud
You can convert ebooks. Authors focus on writing while you convert books to ebooks.
You must be a keen reader and good writer to be a book blogger. You'll need technical skills to set up and maintain WordPress or another CMS.