Generally, such a sentence would have fewer chances of being used naturally. If one wishes to say "Peter reads a book", we'd say पीटर एक किताब पढ़ता है whereas if the book has been mentioned before we'd say something like पीटर वह किताब पढ़ता है (Peter reads that book) to emphasise that particular book which has been talked about before.
This one is just a grammatically correct sentence that can have both of the accepted translations.
So, it appears that the issue was the way the devanagari script was rendering on my computer. Nothing above the line appeared. I don't know if duolingo made a change or I did something on my side, but it finally changed so that I could see enough of it that I could figure out what was what.
"Peter reads book" is grammatically incorrect in English. You need either "the books" or "books" ("a book" would translate to "ek kitab", in Hindi, I guess). On a separate note, do you happen to know good resources to learn Urdu? I learn Hindi on Duo in order to be able to learn Urdu later.
"EK"means one. Please read my explanation above that I am re-posting here. ...Peter reads book...should also be acceptable. Here a native Urdu speaker trying to learn Hindi script. In Urdu and Hindi an article is almost never used. But I must admit that the correct translation would be Peter reads THE book.