Where is the passport book number and what is the difference from the passport number?
For the US department of state visa application, there is a question about the passport book number, which is apparently different than the passport number. What is that referring to? The explanation in the side bar was not very clear and other searches gave answers that seemed to say it was the same as the passport number.
It would be helpful to have an answer that shows a few example passports from different countries with the book number location marked. I'm also interested in understanding exactly how these numbers are used in correlation with the passport number.
This may (or may not) be one in a new Canada passport. The number, EDH02870 in that example, is punched through each page in the passport but is not the passport number (the latter is always two letters followed by 6 numbers). I assume that is the "book number", though I don't think it is identified as such in the passport.
For UK and US passports the book number is the same as the passport number.
For other countries it is not the same thing as your passport number; and you can see in the sidebar - "You may or may not have a Passport Book Number on your passport".
If you have one, it will state so in your passport's identification page (the page with your picture on it); or on the page immediately following it.
It is sometimes called booklet number; as it does on my passport.
For those countries in which a book number is different, how and for what purpose are these two different numbers used?
It is an extra layer of identification to prevent fraud / counterfeit documents.
Are there any countries that _don't_ number their passport booklets? I would expect that the _real_ difference is that a few countries _also_ have a permanent citizen's identification number that they disguise as a reusable "passport number" -- but most just have the _actual_ passport number (i.e. booklet number) and may display a national ID number separately from the passport number.
It is definitely the exception and not the norm. Pakistani passports have: Passport Number, Citizenship Number, Tracking Number and Booklet Number on the ID page; and on the second line of MRP portion they have the passport number, birth date, and citizenship number encoded.
You say book number is the same as passport number for UK/US, however the form has a "does not apply" checkbox. Shouldn't you be advising to select that on passports that don't have a book number, rather than saying to put the same number in twice?