Presumably the "program to generate the unique word list for the entire book" created the list with the separate 'Book, book, Books, books' entries, which you then used as a concordance to generate the Index. Before doing that last part, you really should have concatenated these list entries into one. At this point, you could use Find/Replace to look for all XE fields containing 'book, Books, books' and replace those with just 'Book', thus:
Find = XE book
Replace = XE Book
Find = XE books
Replace = XE Book
After doing that and updating the Index, the 'book', 'Books', & 'books' entries should all appear under 'Book'.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]