How do I search within the kindle unlimited catalogue? e.g. do a search across all kindle unlimited books for the keyword phrase "oak tree". I don't want to simply browse the catalogue by category - I want to see everything in the catalogue related to my search term.
I just noticed that the check box "Kindle Unlimited Eligible" only appears on searches for certain keywords e.g. for "oak tree" it is not available. For "deep learning" it is available. This is rather annoying - it should be available for all searches.