Thanks. I was worried that perhaps I had a crippled version of Sigil, and it would only do say 20,000 words of text. If that's not the case, what are the possible reasons for a file's being truncated?
About the blockquote: I found a case midway where <blockquote> appeared twice before an indented section. I eliminated the extra tag and likewise the closing tag at the end of the truncated file. That doesn't seem to have been my problem.
Should I validate my xhmtl/html before opening it in Sigil? I don't see any problem where the truncated book ends--all the paragraphs end with </p> etc.