Substantial portions of this document, notably the bits on maintaining your UPS, were originally part of the apcupsd documentation. The project maintainers have graciously permitted me to re-use them here. Other parts were part of my Unix Hardware Buyer HOWTO.
There was a previous UPS HOWTO by Harvey J. Stein, last updated in 1997. It was so out of date that I ended up using none of it.
There is UPS FAQ which is slightly dated but still contains some good advice.