I finally broke down and bought a Kindle book. I don't approve of this behavior, but it was a book (Among Others) by a writer (Jo Walton) I've enjoyed reading free (on tor.com and from the library) quite a bit, and it was well reviewed by a number of people I trust. And it hasn't yet appeared on Kobo, and my account on Barnes and Noble is so well protected I can't log into it. (I seem to have used some odd password, and when you ask them to let you change it, they don't give you enough tries to guess the capitalization and punctuation you used for the city of your birth before disabling the account.)
I have yet to succeed in breaking the DRM, although I have hopes that there's some combination of the tools in this article which will do it for me.
So it isn't quite the same as buying a book yet, but I can read it on either a linux laptop or my Android phone.