Another question about combining

[Donna]

My question is about combining subsequent editions of a non-fiction book that each new edition is slightly different from the first. Would each edition be separated individually or should all editions be combined? The example I came across is Herbaceous Perennial Plants: A Treatise on Their Identification, Culture, and Garden Attributes & Herbaceous Perennial Plants: A Treatise on their Identification, Culture, and Garden Attributes just as an example. Just from reading the descriptions at Amazon.com I can tell that significant edits & additions have been made with each subsequent edition (there are at least 4) I don't have access to the actual books, so I can't tell for sure, but I assume the basic information in the books is the same.
Here is part of what Amazon.com description of Fourth edition says: . . .The fourth edition differs significantly from the third edition (2008). Dr Armitage has added dozens of new species and hundreds of new cultivars from breeders throughout the world. . . . Other significant additions have been the detailed discussions of changes in plant nomenclature and in-depth dialogue on the problem of invasive plants. The new edition provides easy-to-read information for over 500 genera, species and cultivars used in the gardening and landscape trade. It also includes Dr. Armitages's interesting and occasionally controversial thoughts on the Internet, pronunciation of plant names, differences in gardening in the South and North, along with many other topics of interest.

Thanks for any advice you can provide. I am fairly new to the volunteer Librarians, so I don't want to combine books that should be kept separate.
~Donna Lundy