Something sui generis is unique. If someone writes a sui generis book about whales wearing hats, then it’s the only one out there.
The adjective sui generis is Latin, meaning literally, "of its own kind." Anything sui generis is its own thing; there’s nothing else like it. The Titanic was a sui generis ship because of its unmatched size and opulence as well as the preventable way it crashed into an iceberg and sank. Every person has a sui generis fingerprint, which is why thieves wear gloves.