what's the difference between "kitabi" and "kitap?"
Read this post: https://www.duolingo.com/comment/7738396
It explains everything :)
"Kızlar turuncu kitabı okur", is also true. You don't need the plural twice in the sentence.
Can you explain why you use the R-present here?
It is a general statement and is not talking about current on-going action. :)
How can you tell it is Girls and not The girls???
Only from the context, so in this case, you can't.