Blushing :shame:
That's precisely my view. As you prepare a book, how often do you notice aspects of images that you'll want to change? I'd contend that it happens a lot and that the book preparation process inevitably involves a lot of back forth. Doing it all in LR makes this trivial. Sure, I can set up a Publish service to supply images for a 3rd party app and to manage the changes, but it means separate copies of photos (esp raws) and I'm constrained to a rather linear workflow.
A second thought is that it helps to do a lot of playing earlier. So I like to shortlist photos with a collection in Library, use stacking to group them into chapters or sections which I may reorder simply as stacks (rather than having to select photos x-y) . Doing this organisation and narrowing down the shortlist in Library seems more effective than in Book where you're going to be distracted by laying out the pages.
Lastly, Book's features have come on a lot and there's a lot more freedom about how you lay out photos. Rather than jump straight to a third party app, shouldn't you at least try to do it in LrC Book (and ask questions) until you're sure it really can't do what you want?