The
Quality Check plugin can search the content of books (it maybe ePub only) - my experience is that it can be very slow.
The
MultiColumnSearch plugin features full text searching - I haven't used it.
I use X1 and Windows Desktop Search (WDS) to search a calibre library, I then use the
Drop Search Results plugin to create a Reading List, which I can cull based on metadata.
X1 and WDS can search the contents of a calibre library folder of 100,000+ books in 2-5 seconds (the indexes they use are maintained by background service tasks when books are added, changed, moved, deleted etc), and culling a reading list by say a Tag value is more or less instantaneous.
X1 and WDS can also search multiple calibre libraries, emails, documents, spreadsheets etc in the one search.
BR