I don't know why B&N isn't carrying it at the moment, but a check on inkmesh.com shows that it's still available at Sony, Kobo and a few other sources compatible with the nook.
I would also point out that Atkinson's 3 books (Case Histories, One Good Turn, When Will There Be Good News?) are actually a series, and should be read in order, so maybe by the time you're ready to read book 2, it'll be available again.
I read the first two books in the series in hardcover. The third was as an ebook (borrowed from the library). They were all fantastic.
If you do a search for the ebook now, it doesn't show up. My best guess is that the ebook was available previously. They removed it to make a change or correction or who-knows. And the website wasn't updated for a bit. Hopefully, they'll get it back up soon.