IANAL, and it will depend on jurisdiction. But generally transformative uses that are a completely different application, and don’t compete with the original are likely to be fair use. A one-line summary is probably more likely to promote the full book, not replace it. A multi-paragraph summary might replace the book if all the key messages are covered off.
How lossy can it be until it’s not infringement? One-line summary of some book is also a lossy reproduction
IANAL, and it will depend on jurisdiction. But generally transformative uses that are a completely different application, and don’t compete with the original are likely to be fair use. A one-line summary is probably more likely to promote the full book, not replace it. A multi-paragraph summary might replace the book if all the key messages are covered off.