Kto napisał Książkę Miłości?
...or should książka be in the nominative since it is part of of a proper name? ('The Book of Love' was a movie in the 'American Pie' franchise).
That suggests finishing writing something specific. Here, my friend used to write about love, generally.
pisal is the past imperfect
which tense is Mój przyjaciel napisał o miłości
would that then be the past perfect
my friend wrote about love