[mrec]

I think they're looking for something that searches content. Calbre search only covers metadata.

Need to clarify re ebook format though - if it's DRMed then any third-party tool is going to be out of luck.

[newman8r]

Someone was working on a full text search plugin for calibre https://github.com/idealist1508/Recoll-Full-Text-Search-For-...

I intended to try it out but never did - might give it a look again soon.