[PKTC]

Thanks for the various replies.

I've worked out all the problems with converting my files to EPUB2 and 3. I can validate the resulting Epub2 with Epubcheck successfully. After working on EPUB3 with Sigil for a while, I got to the point of no errors at validation but I am getting this type of warning, with one warning for each line of the manifest that lists the document sections:

WARNING OEBPS/content.opf 13 140 XHTML Content Document file name 'OEBPS/Text/Practical_Guide_to_the_Two_Truths.htm' should have the extension '.xhtml'.

I have tried going into the content.opf file and manually changing each of the lines per the warning message, but that makes things worse, producing errors at validation. I have tried rebuilding the manifest using the option for doing so in Sigil but that just gets me back to the set of warnings explained above. Any ideas on how to solve this? If I can through this, I should be done, I think.