Very interesting blog post over at Dear Author about customising car dashboards, and smart phone displays, and, eventually, ebooks. I agree with everything Jane says. The comments are also interesting–particularly the one about customising book files for printing via Espresso POD machines. I don’t know if publishers do this yet, but they should. Imagine if you could print out the book any way you like, perhaps even personalise by adding photos. (But then I imagine one would run into copyright knots.)
Does anyone out there have experience of Espresso and its custom options? I don’t know enough, clearly.
Nicola Griffith 
I haven't used Espresso, but you know that is the way I tend to lean; for me, ebook wise? What I really actually practically want is what you are describing; customizable mix n' match. Especially for roleplaying games. I want to be able to take the relevant rules as they apply to one character & make a custom document. Yes.
I have no desire to format or personalize books I read. I am reading for the authors voice. I want to read it the way she thinks best gets her story across.
I do agree that formatting, etc. impacts how the story comes across. I've experienced it myself with paper vs ebook. But I am paying for them to figure that out. As that commenter said, “There are professionals trained and experienced in typography who know that font, hierarchy and structure do make a difference in the reading experience.” Let them do their job.
And yes, please focus on getting basic ebooks right before messing around with enhanced ebooks.
Being able to choose things like how to sort the books – by author, title, etc. is a different thing, and I'm all for that.
Maybe after I'd read the book, I might be interested in adding something to it before showing it to a friend, but mostly I think not.
A book cover might entice me to pick up a book (or turn me off), but the first thing I do when I start to read a hardback is pull off the dust jacket.
I don't really want fiction customised. I want to read it in a boring, standard layout with boring, standard font, on my Kindle. (Though every now and again I'd dearly like to fix some idiot's notion of justification.)
Non-fiction, though, especially of the kind of public-domain stuff I use for research, and prefer to have in print form, that would be cool. The more I think about it, the more I think there's no reason it can't be done. In fact, it probably is–is one bothers to do the work of collating and setting up the file and then trogging it across town to the nearest Espresso machine. (Because, hey, just zipping it there through the ether would be nearly as much fun as watching it emerge and drinking coffee with the people who run the machine, etc. etc.)
Jennifer D, the customizable stuff is going to revolutionize textbooks– well, the ones you decide you want hardcopy of (reference) rather than electronic. Or like Nicola says on homebrew stuff; If I want to read page after page, I want to read it on a page, not a screen.
My partner is (legally) blind, and adores her Kindle–because she can make the text large enough to read. The Kindle has made reading really accessible to her for the first time since her vision worsened a number of years ago and she was no longer able to read regular text without adaptive tools. (Though her giant magnifier machine is lovely, it's really difficult to curl up on the couch with it, and it's not pleasant to read anything of any length with it.)
I think that's one of the real benefits of customization–letting those who need larger print or other changes access a greater variety of books. I'm personally very lucky in that the things I need adaptive technology for I can either get relatively easily (TV closed captioning) or can live without (podcasts, audiobooks). But I'm all for anything that lets more people access content in more formats and more places.
Yes, there will be more customisable everything as the baby boomer generation ages. And that will benefit us all.