Personally, I find monospaced, serifed fonts to be the easiest to read, and am frequently frustrated by the lack of books that use a monospaced font, and I wonder who is to blame for it.
In all of the software usability testing that I've done over the years (mostly involving web applications), monospaced fonts consistently score the highest for readability and comfort with the users.
Actually, I'm fairly certain that a majority of people find proportionally-spaced fonts easier to read for paragraphs of text (though not for source code, for example). That's true for print, and I doubt that it's different on the Web. Alas, I couldn't find a good reference via a quick Google search. PLEASE NOTE that I completely respect that some people fine monospaced fonts easier, I just think they (and you) are in the minority.
I would bet that the reason publishers like condensed text is the same reason they indicate new paragraphs by indenting the first line rather than separating adjacent pars with a blank line; the same reason they will break a long line of metrical drama and append the tail to the end of the line above or below; the same reason they will publish a long narrow list as a two column table even though there's no connection between the left and right halves of the content on any one row: paper costs money, and they want to get as much use out of each square inch as possible while keeping the text more-or-less readable. They are largely happy to sacrifice some readability if it means they can limit the amount of paper they have to use.
I completely disagree. There's a huge variety of books (and magazines) with vastly different print sizes and densities, and aimed at different markets. If monospace were easier for the majority of people, some (probably most) publishers would use it -- at the very least in specialized niches. To cite an extreme example: children's picturebooks have plenty of room on the page and larger print; why aren't they monospaced? How about large print editions? Expensive journals? Or, look at books that specifically choose more expensive paper to make an impression or cater to an audience that appreciates it. -- Cheers, Scott S. Lawton http://Classicosm.com/ - classic books http://ProductArchitect.com/ - consulting