Two books or three?

Hi!

This is my first post on this site, so I hope its in the right section.

I've asked this question around and no one seems to know, so whatever information you might have, I'm interested.

Here it is: have you ever heard or come across a set of books (two to be exact), which were published as volumes one and two, but had approximately a third (!) of the same content. This third is an integral part of each book (with very slight variations, associated to the particular topic of the volume).
I think the logical thing to do is to publish a third volume, which connects the other two, but my esteemed colleague (the author) disagrees. I suppose one might be able to find similar practice in certain technical books (which these two are in a way), but I have no way of knowing, so I have to ask.

Anything you may know will be much appreciated.
Thanks!

Thanks for the feedback so far everybody.

They're both non-fictional, technical books (its actually specialized literature, similar to what one might find at university).

No, not self-published. She's trying to get this double out, and I must say I'm not entirely surprised that she hasn't found a publisher yet. I've never heard of anything like what she's planning (I'll be sure to mention you James, when I talk to her about it), and I'd agree with Jenna that its absolutely irritating to get the same information in two books, which are not thematically connected (but do have this one very important aspect in common).

So, what I'm thinking is that she should change this idea of having the same content in two books, and make a seperate, third book, instead of having the same content, a third (quite a bit, when you consider that its 120.000 words long), in each of the initial books .

But if anyone has heard of anything like this - maybe come across it during their education. I can't completely dismiss this yet, so I'm thinking that schoolbooks are a place to look for something like this, before I tell her she's gone absolutely, raving mad and should seriously reconsider her concept.

Sorry for the outrage I seem to have spurred up.

I may have misunderstood to whom your "disrespect" is connected to Anthony, but I think it might to my author friend for not doing her job the way she should be - which is also why I'm asking this question. I'm not a writer (not books anyway, just column here and there) so I don't have much experience in this field, I'm just asking questions on my friend's behalf so she doesn't make a mess of things. She likes to be secretive about this piece of work, and I respect that secrecy by not giving out too much information.

Its multiple manuscripts (two), being published as a double or a "doulogy" of sorts, but I don't think you could a call this (technical) piece of work a duology. Both books will be on sale together, but will be available separately.

The market segment overlaps just a little bit, but not much (they're being sold together), and this third of both books is actually quite the same thing - not like there would be similar bits here and there, its actually an entire chunk of both books (a couple of chapters - together) that are quite the same (just an odd word or sentence here or there is different, so that it fits to the theme of each book). It is this that I find quite baffling, and I think its so senseless that even you can't quite grasp what she's doing, hence the reason for our misunderstandings so far.

"Call me arrogant, but it looks like they should finish the manuscript, polish the heck out of it, & meantime be querying publishers."

The books are done. One has been translated and the other is in the process. Publishers are being queried, but I don't think the concept is ... well, simply good enough to be ready for publishing. I think that the readers are being made fools of, and I don't see how anyone would want to publish this in the form it is now (there's still time, because the second books hasn't been translated to the end yet, so no one has had the opportunity to read it yet). The actual content is unbelieveably good, quite revolutionary, its truly amazing (not just my thought, this is the thought of several renowned experts and prominent academians), but the impact will be ... I'll put it like this: instead of a massive crater, she'll just be scratching the surface. I think its a waste, and over something so superficial.

I hope I've been specific enough, and I'm sorry to have caused anyone any discontent. Thank you again for your replies.

Thank you, it does. I think I'm beginning to find out that my initial concerns were well founded and that she should definitely change her concept.