Which edition/version should I use for citations with page numbers How do I cite a very old and well-known book which has multiple editions and reprints, various editors, facsimile reprints, and a recent paperback version, plus it's even available free online, again at more than one site. The text I want to cite is common to all of them so that's not an issue but I'd like to be able to give page numbers in the standard way. I don't know which edition to cite. The actual copy I've been using is a hardback 1970s reprint of a 1900s edition from the university library but it's not necessarily easy for others to find.
Things can occasionally get tricky, but most situations are handled by these guidelines:


Cite what you've seen.  If you have one edition, don't assume that another has the same numbering (for pages, chapters, sections, equations, or whatever) without checking.  It's always better to cite an out of date edition correctly than to risk giving an incorrect reference to another edition.
If you've seen several editions, try to cite the one that will be most useful to readers.  Recent editions are generally preferable, both because they are easier to find and because they may be more up to date in other ways.  (But if a book is out of copyright and reprinted by many publishers, then recency may not be salient.)  If you know that a recent edition is not useful (for example, because material was cut), then you should say so.  Translations may be preferable for readers who do not speak the original language.
If you can't tell which edition would be most useful, then the decision probably doesn't really matter.  The most recent edition is generally a safe choice, if you can get your hands on it to check the reference.
More detailed references can help disambiguate.  A quick numerical reference is efficient, but it becomes near useless if the numbering has changed, while adding more context can make it easier to figure out where it might have moved in another edition.