@
phossler - as I understand it, you want a One-Click something that marches through a book detecting all the 'first paragraphs' (some book's format the paragraph following a scene break as a 'first paragraph') that have leading Upper Case words and then change the leading Upper Case words to Sentence Case words.
I've yet to find a Case changer that deals with proper names correctly. So '
SMILEY FLEW TO VIENNA; CARSTAIR'S was in the arrivals hall to meet him...' inevitably becomes '
Smiley flew to vienna; carstair's was in the arrivals hall to meet him...'
Some errors may be picked up by a Spellchecker. But not if '
SMILEY FLEW TO DODGE CITY; CAPABILITY BROWN was in the arrivals hall to meet him...'
I suspect a goodly number of 'first paragraphs' in novels, biographies, and histories contain a proper name in the first sentence. So even after a One-Click button waved its magic wand, you'd have to eyeball each 'first paragraph' to sort the wand's curses from it's blessings.
I always found the capitalisation of leading words in print irritating, especially as it's often arbitrary with no regard as to the author's phrasing. To me they're a printer's affectation.
Getting rid of them in e-books. whilst tempting, usually requires more effort than I'm willing to expend. Unless of course the book is in need of serious tin-bashing, in which case selecting the text, changing the case, and correcting proper names may be worth the effort.
BR