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MarjaE - the Quality Check plugin has a content search, but IIRC it has to convert books on the fly so it's slow.
The Multi-column search plugin also has full text search, IIRC you have to convert books to TXT to use it. I believe you can combine metadata searches with text searches, e.g. tags:"=history", and book contains "waterloo" - not neccesarily with that syntax
BR