Ah, the infamous "It doesn't work" complaint. Darned hard to troubleshoot, isn't it?
I can tell you this much: NOOK uses Adobe Reader Mobile software for EPUBs, so there shouldn't be anything really specific to NOOK. The same thing should happen on a Sony or a Kobo or any of the other zillion e-reader models that use RM.
And it should be pretty similar to what you'd get in Adobe Digital Editions, which you didn't say you'd tried. It might be worth giving that a spin.
You mentioned the cover art, and I have seen quirkiness on NOOK with oversized cover art. Mainly it presents as weird pagination, where the cover might appear on multiple pages and there might be a blank page or two after the cover. That alone can be enough for some readers to proclaim that "it doesn't work" because they didn't try clicking the page-forward button a bunch of times. (I've been fooled by that myself.) NOOK doesn't provide the entire 800 pixel height for RM to display the e-book in, so your cover art needs to be a bit less than that.
If I remember — at my age that's far from a promise — when I get home tonight I'll try out the samples of those e-books and see if they work OK on my NOOK 3G 1.5.
Oh... one other thing that can cause a general "doesn't work" complaint: if the reader downloads the EPUB from his B&N library to his computer using the Download button, the B&N website gives the file a name like xxxxxx_epub.v2.epub. For reasons known only to the NOOK development team, NOOK 1.5 software no longer accepts EPUB files that have more than one dot in the file name. If the user sideloads that EPUB without changing the file name, he'll get an "unsupported format" error. This will happen on all EPUBs that the user downloads that way, not just yours. (Simply renaming the file before sideloading fixes the problem.)
Okay, I can confirm that the free samples for all three titles open to the cover art, then after about three seconds the NOOK automatically returns to the library listing. If I attempt to page forward from the library art, it pages the library listing instead. This is on (E-Ink) NOOK 3G version 1.5.
I'm going to investigate a bit further to see if I can find out what's up. I'll post what I find.
Update: This is definitely strange. I've run the sample of Expectation through Sigil, cleaned up the table of contents XHTML file (the sample lists all of the chapters even though they're not all present in the sample), and gotten it to pass Flight Crew with no errors. And still it won't stay open.
I've tried to open that sample in Adobe Digital Editions, and ADE shows the cover page but crashes when I try to go to the next page. This happens both with the original sample and with the cleaned-up sample. It happens if I try to page-forward and if I try to jump to a chapter from the table of contents.
So at least there's something you can work with: use Adobe Digital Editions as your test case.
I'll keep looking, but I sure don't see anything out of whack that should give Adobe's EPUB renderer such heartburn.
Second update: I commented out everything on the spine ahead of the dedication. ADE now shows the dedication page, and crashes when I try to move away from that page. ?????
Found it. The "font-weight" values in the CSS file are invalid. They currently read:
They should read:
Remove the quote marks, and uncapitalize it. Then you should be ready to go. That made them work in Adobe Digital Editions and on my NOOK.
Be sure to make both corrections: quote marks and capitalization. There are three lines in the CSS files on those samples: for H1, H2, and H3.