It’s a toggle. Click on the % and it will magically turn into page. Click on the word page and it will change to %.
Using the percentage completed can be easier than page numbers when reading an ebook or listening to an audio book. Ebooks are supposed to default to %, but any edition can accidentally get changed to %, since it is just a toggle.
You might want to verify that you have shelved the same edition that you are reading when you start a book. Different formats often have different numbers of pages. You can change editions from the book page by clicking edit beside the format or edition field.