If you haven't, I highly recommend taking a look at Knuth's manuals. Even if you are completely uninterested in METAFONT or TeX, they are a delight to read and

If you haven't, I highly recommend taking a look at Knuth's manuals. Even if you are completely uninterested in METAFONT or TeX, they are a delight to read and I would hold both up as a very good aspirational target for software manuals.