I think it is insensitive ba. Most of the time for birthdays,
the intention is to give a gift that would please the person and
make him happy.
I think you could have discussed the war with your friend, offered
to get him the book (or other non fiction books), brought him to
the museum etc, on any other day than on his birthday. Still, you
got to remember as you said, that many young Japanese have no idea
what happened. Also it is not their fault, they shouldn't need to
carry the guilt of their forebears.
Educating and letting them learn something new, should also be done
with a degree of sensitivity. To rub it in right to his nose on his
birthday is not something very polite. I am sure you won't enjoy
having someone give you a book which suddenly lets you know your
family/race was guilty of genocide or massacres on your birthday
either.
It's been more than 50 years, it is time for education AND
reconciliation.
