While reading a Robert B Parker ebook last year, I noticed this in the front matter:
“V3.0_r2”. To my systems eye, this means version 3, release 2.
It struck me as a little odd to have a version/release number at first. But then I thought of it like a printing history rather than something more traditionally associated with version history. Something like software perhaps. Or Star Wars movies.
(I got a whole post somewhere in my head about how Lucas didn’t treat his movies like movies, but more like software releases. Maybe someday it will get out of my head and onto the page.)
I’m not sure yet that adding version numbering to your ebooks matters. For example, if I learned that this particular book was now in release 3, or version 4, what would I do with that info? Would I email or call the publisher, angrily demanding the latest version?
Probably not.
So what does this mean to you (or at least, to me) as a self-publisher? I think for now it means take it or leave it. In my books, I chose to add it. Again, not sure what I’ll do with it. I suppose if I decided that there was some major textual screw-up that needed fixing, I might increment the version and off those who purchased the faulty previous version an upgrade.
Except, how would I confirm their purchase?
Plus, of you’re putting out a new revision of a self-published ebook, Amazon lets you label something as edition 1 or edition 2. It’s right there in the “wizard” Amazon uses to guide you through the publishing process.
Like I said above, I can’t see how this matters to anyone but the person putting the ebook together. In my case, that’s me. So at least I’ll always know which version I’ve released online.
Scott C Lyerly 
Are you saying that v3.0_r2 is more updated than v3.0? Do you have a source for what these numbers actually mean? I have been looking for ages to find out if I should prefer one over the other. If the “r” numbers are revisions of the pre-release before the final v3.0 is produced, then I should prefer v3.0.
Put another way, if you are working on a third major version, do you start off with v3.0_1 and work on up until you are satisfied all the required stages have been completed before you finally label the final version merely v3.0? Are the “r” numbers indicating beta versions, perhaps?
I would really like to see what the official explanation of these numbers is.
I think it’s up to each individual publisher how they want to version a book. Mine tend to go very straight upward, 1, 2, 3, etc. But I’m writing fiction. A nonfiction book may align versions with editions, with minor formatting correction as minor version numbers instead of major ones, like 1.1, 1.2, 1.3, and leave the major version changes for new editions with expanded material.