@elibrarian: Thanks for the tip on the LibreOffice extensions.
Meanwhile, I've started looking into a particular problem of mine.
"My" ebooks are mostly 19th century Norwegian books, using spelling and grammar that's somewhere half-way between Danish and modern Norwegian, meaning I can't use spell-checkers, because no pre-1907 Norwegian ispell dictionary exists. However, a lot of proper proof-read digital 19th c. Norwegian texts exist (>10,000 pages).
I came across this 21-line(!) spell-checker at
norvig.com, the site more famous as the origin of the
PowerPoint version of the Gettysburg address
. Based on a huge reference text, it checks spelling of a word. Though oriented towards human errors like transposition, it should be possible to tweak it to look for typical OCR mistakes, like 'f00lish' and 'junip'