Because of limited shelf space, many libraries or private diving physicans may sell or trash the older editions to make room for the newer edition.
For my purposes, only the latest will do.
I'd be happy to part with old versions, but I keep one past edition in other offices as ready reference.
However, diving medicine hasn't been marching forward as quickly as other fields of medicine, and 60% of the old version is still established: decompression protocols, oxygen toxicity, DCI, CAGE, fitness to dive etc., are roughly unchanged. Of course, one can never know if a 1993 controversy or unknown has been better understood in the 2003 edition.
However, a free book or cheapo book versus paying $150 for the latest edition maybe a compelling way to get a detailed glimpse at an authoritative diving physiology reference for low cost.