I'm wondering how to reference a book The Jepson Manual, 2nd Edition shown below.
Baldwin, B. et al. (ed.). 2012. The Jepson Manual, Vascular Plants of
California, Rev. 2, 2nd ed. University of California Press.
I wondering about citing the revisions which are available on-line at eflora. There are now two revisions of this second edition. Should the revisions be listed in the citation as above or some other way? And should I keep the book date at 2012 even though the online revision 2 was 2014.
There are two basic approaches to citing a "live resource" of this type, depending on whether you are citing an aspect expected to be durable or that is expected to be particular to that particular version.
To cite an aspect that might be particular to a particular revision, e.g., a text quotation taken from a certain entry in the manual, you should cite in just the way that you have shown (modulated for the particular referencing style). In this case, a good way to think about it is that "Revision 2 of Second edition" is just a fancier edition number that includes some sub-structure.
To cite an aspect that is expected to be durable, e.g., California having a high diversity of plants due to the high degree of variation in geography, it is more appropriate to use the citation form for an online resource, where you note the date retrieved but might not bother reporting the particular version, since the reader should expect that future revisions will still be just as valid sources of the same information.
If in doubt, give full details. That way, your later reader who needs to check will get whatever the current (or available to them) version might be, and in case there is a relevant discrepancy will be able to track down your exact reference.