There is one book that has been on every composer's shelf (and desk) longer than any other. This one:
I just scanned the cover of my copy. I haven't owned it for my whole career, but I have certainly been aware of it and worked with selections from it in other texts for a very long time. Some version of this book has been in print since 1764, which probably makes it the longest-lasting music publication ever. It is currently available from Amazon for $8.54. F. W. Marpurg began collecting the chorales and the job was carried on by J. S. Bach's son C. P. E. Bach. A major revision was made for the third edition in 1831 when the upper clef was changed from the soprano C-clef to the more familiar G-clef. There were a number of further editions and revisions. The currently-available edition, pictured above and available through Amazon, was edited by Albert Riemenschneider and published in 1941.
The stimulus to mention this book was a couple of comments left on an old post of mine talking about Bach chorales. Go and read the post and the comments. Not long ago composer Farcry C. Zuke left a link to his fascinating discussion of the same chorale I talked about. Please go and read Farcry's analysis. He says "The harmonic oddities of Bach often involve stepwise motion." I have put this in a slightly different way, saying to a friend of mine once that you can get away with almost anything harmonically if you lead into it by step. Farcry goes through the whole chorale pointing out a number of interesting aspects, concluding that the chorale "is actually a melody in E Phrygian and Bach has harmonized it in A minor." Yes, and ended many of the phrases, including the final one, with a half-cadence!
Just for fun, and, I guess, to show how hard it is to set out to harmonize a chorale like this, I took the melody to Aus tiefer Not and did my own harmonization. Sure, it's clumsy and not in the same league as Bach. But I didn't spend too much time on it. I like a couple of the ideas and stumbled across at least one interesting cadence. I was just trying to see how much I could get away with, sticking to stepwise voice-leading. Let this be a lesson to all of us! And hey, when you, inevitably, diss my harmonization, be kind.