i love this

BookkeerperSchema.hs

instance ToSchema (Book' '[]) where
    declareNamedSchema _ =
        pure . NamedSchema Nothing $ mempty
            & type_ .~ SwaggerObject

instance (KnownSymbol k, ToSchema v, ToSchema (Book' xs)) => ToSchema (Book' ( k :=> v ': xs )) where
    declareNamedSchema _ = do
        valRef <- declareSchemaRef (Proxy @v)
        let key = Text.pack (symbolVal (Proxy @k))
            rest = toNamedSchema (Proxy @(Book' xs))
        pure $ rest
            & schema . properties %~ (at key ?~ valRef)

{-
λ> encode $ toSchema (Proxy @(Book '["foo" :=> Bool, "bar" :=> Int]))
"{\"properties\":{\"foo\":{\"type\":\"boolean\"},\"bar\":{\"maximum\":9223372036854775807,\"minimum\":-9223372036854775808,\"type\":\"integer\"}},\"type\":\"object\"}"
-}