....tell you a lot about the history of your parish, and even of the trends in church music and American culture as a whole (how 'bout some of the 70's artwork on those covers?!).
Since I've been at Bethany, we've "archived" close to half of the choral music. Evidently it is some sort of transgression to throw the stuff away, supposedly because of all the needy congregations out there just aching to sing dated, heterodox drivel, so we box it up and store it in the attic above the gym in a back corner - where it can be disposed of by a future generation. Some of it actually does get thrown out, such as illegal photocopies or old evaluation copies of out-of-print octavos. But mostly we just store away those things we know we will never use so that we can make room for that which we can and will use. And now we are looking at instrumental music as well.
What sorts of things does one discover? Periods where the all the new music for choir was the "praise music" of an earlier generation, a type of "traditional" CCM, from many of the same publishers who now give us choral knock-offs of today's Bapticostal radio music. Other periods are much better, with sacred classics from the Lutheran repertoire. Periodically, one even finds a few years where the director purchased psalmody and hymn concertati. And then there are those romantic eras where a predecessor bought lots of big works (children and adults' choirs plus brass and bells!) which are still in pristine, unused condition. Guess (s)he went off to a workshop and got all excited about something.......only to discover that planning, rehearsing, and putting together all these forces in Bethany's old sanctuary was too much to do. I found a similarly interesting history when I went through the library during my cantorate at Trinity-Peoria (with choral music from the 1890s in German, along with English anthems dating back to the 1910s!).
I think digging through the library is one of the most important things a new director can do when coming to a parish. One can see where a choir and a congregation have been. One can discover some favorites that will help the new director win the confidence of his new choir. And one can find gems along with the jokes. (I'm sure those who followed me at Trinity have enjoyed the copies of "How?", the spoof of Carl Schalk's hymn anthems written by the late John Folkening.)
What have you found in your library?