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ColMac - if your books are either US spelling or UK spelling (i.e. not mixed within a book) try switch dictionaries
Edit->Preferences->Editor Settings->Manage Dictionaries.
If you have books with both or you want a swiss-army-knife to cover all books then there are some EN dictionaries at Apache that carry both spelling variants (e.g. 'spelt' and 'spelled', but only 'dispelled' and 'bespelled'

). They're rather big so they may have an adverse effect on performance when they're used.
BR