I've been rereading a bunch of David Weber books, and I know Baen has the first few in each series free on their site. March Upcountry might be a good one...not as complicated as the Honorverse books, so easier on those who don't have as much time to read.
I read "Red Mars" and the second book (I think "Green Mars") long ago and I remember not being overly impressed but at the time the books may have been a bit dry for me. I hesitate to do a trilogy book at the moment but we have some good names on that list. Stephenson is another I'd like to read as well.
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Since I've already read it, if you give me an idea of how many pages/what percentage of the book you'd like to do for the first week, I can suggest a stopping point without anyone seeing any spoilers as they skim through for chapter breaks.