While that article doesn't seem to go into any detail as to why it was known to the police by that name, the different phrasing of "the suicide's handbook" vs. "suicide handbook" probably means the police referred to the Rubaiyat as "this specific suicide's handbook" rather than something of a guidebook for suicide in general.
https://trove.nla.gov.au/newspaper/article/130362134?searchT...
While that article doesn't seem to go into any detail as to why it was known to the police by that name, the different phrasing of "the suicide's handbook" vs. "suicide handbook" probably means the police referred to the Rubaiyat as "this specific suicide's handbook" rather than something of a guidebook for suicide in general.