@
DrewBoyd - if you don't already have it may I suggest keeping
Pablo's EPUB tutorial at your elbow.
And there's a lot of relevant material in the MR Wiki
MobileRead Wiki - ePub
Added - what did your partner use to create the books?
If it was MS Word or similar then there are tools that will do a decent convert from DOCX to EPUB. IMO these three are worth considering:- Calibre Conversion, Calibre ebook-editor import and the
e-Book Tools Word add-in. The last one has several useful features unrelated to e-books per-se.
Good luck BR