So I popped over to the Google ebooks store, tracked it down for £4.99, checked that it was in epub format (because frankly PDFs do not resize well) and downloaded it. I then had to install Adobe Digital Editions, because that manages the DRM. And I then googled for this guide to stripping the DRM straight off of it.
It took me about five minutes to download the latest version of Calibre (which is the best app for transferring books to readers, in my experience) and the tools for removing the DRM, and everything just worked first time. I now have a file in an industry standard format, that's not locked in any way, that I don't have to worry about people removing my right to use, or locking to any number of devices.
I refuse, if you didn't know, to have anything to do with locked file formats if I can possibly avoid it. There have been enough examples already of people buying music or video from a vendor who then shuts down their authentication server, leaving them with a bunch of files that can't be played. And once I've bought a book/song/video I want to be able to read/watch/listen to it wherever I fancy, without a third party telling me what I can do with it.
And so I'll happily buy MP3s from Amazon (or whoever), because everything plays MP3s. And now I know I can pick the locks on Google books I'll happily buy them and instantly do so. I just wish that (a) they wouldn't feel the need to put the locks on in the first place, and (b) more people would do similarly with video.
Original post on Dreamwidth - there are comments there.