Alright.
Did some more searching. Here is the best I could find.
Below is an old Mexican passport
Notice, there are 2 numbers, one on top, one on the bottom. I am assuming the top one to be a book number. So, if you have a Ukrainian passport with a number that is different from the passport number, that is the book number. They say, if it exists, it should be on the first page, not on the picture page. Likely this does not apply.
The only doubt I have is whether the numbers stamped with holes, on every page or half the book, counts as a book number. I don't know if they still do that. I believe this number is the same as your passport number, question is if it would be considered a book number as well.
Overall, I think this does not apply to Ukrainian passports. I think you would be safe with checking off "does not apply". I am 95% positive, but I have no certification so don't take my word.
Good Luck and let us know what you find out when you submit the application. There is clearly a lack of sources on-line, so this would help to be a future reference.