This is an example of a data source in ESL and English literature lessons. There are many others (a search for “free ebooks” yields more examples), but Project Gutenberg is extensive and has books in different languages.
The library is probably more suitable for more advanced students, as the books are older (that‘s why they are free), but any text can be used for any age, if the task is phrased appropriately.
Students could pick any text to do linguistic work (for example by collecting parts of speech like nouns, verbs, etc.) or they could read parts of a work of literature or whole novels (and analyse the author‘s style of writing), or they could compare and contrast a part of a book in English and maybe their mother tongue to learn more about the two languages individually. Often, free audiobooks are available and could help the reader with pronunciation etc. as they are reading along with the text.
These are some ideas of how free ebooks like the ones on PRoject Gutenberg can be used, but there are many, many more.