There's no reason why LR shouldn't bundle the whole book into a single PDF and I'm sure many people would like this.
Ya, I could combine them in Acrobat, but its a bit annoying.
Something else I just realized. I would like to distribute the pdf as a draft to some friends but just realized that its huge (30MB for a modest number of images). What is the best way of reducing the size? I thought of shrinking the images on export and reimporting them or printing to file and reimporting them. Or I suppose I could do it in Acrobat?