Re: Why is it said "b" for "book", and "p" for "pen"?
Are you talking about words we use to make sure our listener knows which letter we need them to write? If so, there are hundreds, if not thousands, of options. We don't all use the same words.
My name is Peter Parker. That's "P" for pen/piano/planet/pumpkin/porridge/pancake etc at the start of my surname.
(Most people won't have any trouble spelling Peter, but they could mishear Parker as Barker.)
Of course, there is the standard NATO phonetic alphabet which some of us use in all situations.
A: What's your name?
B: Sylvia Plonnetti
A: Can you spell that for me please?
B: Sierra, Yankee, Lima, Victor, Indigo, Alpha. Then Papa, Lima, Oscar, November, November, Echo, Tango, Tango, Indigo.
A: Thanks.