While my situation is with ordering prints, and not books, through Aperture 3.4.3, I thought I'd add my experience in the event that the process is similar or the same and the jobs are uploaded to similar, or the same place(s)
I had a total of 180 picture I wanted to print. This full job failed several times - "Connection could not be . . .Please try again later." I don't remember the exact message, but it is the same as the others receive. Afew weeks agao, I took a batch of 71 or 72, of the 180, and the job completed successfully. Today, I first tried 99 of the remaining prints, which failed, then backed down to 89, and it succeeded. I then sent another batch with the remaining prints and it completed successfully. I checked region and ID info, but not security question responses, I'll admit, and it all looked OK.
I did some quick math and determined that the job that failed estimated to be about 60MB. The job that succeeded (89) was les than 55MB. So, in summary, I had success by reducing my job size, and keeping all other things equal, but am still not clear if it is the job size and/or print size (mine were not mixed, all 4x6) that presented a limitation. Or maybe, still it was something else.
Regardless, for those of you, and me eventually, wanting to send a book to print, if this is a job size limitation, it is a joke. I'm fairly sure I have seen complaints of this sort with iPhoto going back to 2007. The strange thing is, that there doesn't sem to be any definitive answers from Apple, or the media, or anyone about what causes this, and/or if/when it will/might be fixed. Very odd. Maybe it's just me, but since this is supposed to be a Pro App, and Pros would never tolerate this ambiguity, where do they/you all go for this kid of printing instead? I haven't heard much of that either. Very, very strange.