Physical books or e-books? I assume e-books?
If your not trying to do anything fancy and your just mining vocabulary (and nothing else). You could use a website like Book Manager | ッツ Ebook Reader (you can edit some settings to make it work for western/regular books) to read e-books in your browser which allow Yomitan to work with them (though idk how good Yomitan is for spanish, it’s great for Japanese).