I take it this is about a childhood friend who has been with you through thick and thin and somewhere along the line that friendship had an abrupt end...perhaps?

Friend, whose face
has not filled my eyes
in my last three lives,

^^To me this suggests that its about a childhood friend whom you lost at a young age because the words "whose face has not filled my eyes..."

my memories of you
are like broken pushpins
stuck on an old map:

^^This is so vivid! I can see the pushpins with old photographs of places and moments and with time those pictures too have aged yet their memories are still fresh in your mind.

The second stanza though shorter then the first speaks of a story which was to happen but never did happen. You have told in such few sentences it grabs my attention and at the same time a picture of a pair of dare devil boys who were not afraid to face anything stands out.

The ending is touching it leaves me with an image of that book shelf and that lost memoirs dangling reminding about youth, its journey and that inevitable end which we humans have to face.

Nicely penned Sir, feels like I've been reading a chapter from a story.