Thanks for the help. I will try splitting the book again.
I used Sigil to split the book, and I did some editing (tagging chapters for TOC, formatting titles and cover to H1) all using Sigil. I also used Calibre to get basic metadata and a cover image. Otherwise I don't know what I did.
How can I check the OPF? The check by FlightCrew (included in Sigil) says everything is ok. I tried W3C, but I didn't understand what came up.
If there is a place I can learn more about this, please direct me. I have looked at the Sigil User Guide and some of the links from there.
I'll post later with my results and/or resolution to the problem.