I've read a couple of reviews that suggest that the style change might be down to Pratchett's Alzheimer's and it's possible that this is true ... the style could also be perceived as a detailed story outline that never got its final sharpening. But it's consistent from end-to-end and the man is more devious than all his characters put together (has to be ... he wrote 'em) and has already spoofed other narrative styles in the past and what could be more Victorian than steam? So my own suspicion is that it's all on purpose.
I've now got his "The Long War" to read but I think I want to re-read "The Long Earth" first, just to refresh myself on those nowhere-near-as-familiar-as-the-Discworl
P.S. A little diligent Googling, requiring sifting out a ton of hits on Bob Marley, Bob Dylan, and any other famous Bob you can think of and have finally tracked down the author of "Baby Bob" (which I've never known, my mother's copy being one of those by-the-nameless-author-of-other-title editions): Evelyn Whitaker (now off to see what titles of hers are available in digital form)