but all of the ğ and ı display as g and i.
As a second thought, what you are signalling here could be also the result of a defective epub conversion.
Let's suppose that the original source of the ebook was a book digitized with a scanner and converted in editable format via an Optical Character Recognition program.
Then it's quite possible that ğ and ı were mistaken as g and i during the process, a typical issue of OCR conversion caused by multiple factors.
An epub editor like Sigil could help in substituting wrong occurrences with right characters.
I'm deeply sorry, I need to sleep.
You said you're using a txt file. May I ask if you can provide me a link to the file, I'd wish to try it on my PB603. Or attaching just a partial file here?