A new witness in the Diddy criminal case says he has a manuscript belonging to Diddy's late girlfriend, Kim Porter ... but her best friend tells us Kim never tried to write a book.

Lawanda Lane, Kim's best friend and the caregiver of Kim and Diddy's twin daughters, tells TMZ ... she was best friends with Kim for 30 years, including 20 years living under the same roof, and there was never a manuscript written by Kim.

Kim's BFF tells us she was with Kim every day and Kim never wrote anything down ... and she says the family doesn't know who new Diddy witness Courtney Burgess is, even though the guy claims he's one of Kim's former associates.

As we reported ... Courtney testified before a New York grand jury last week ... and he also claims he has sex tapes showing celebrities getting down at Diddy parties.

There was a purported Kim Porter memoir on Amazon earlier this year, but it was pulled after her daughters denounced it and cast doubt on its authenticity ... but now Courtney claims he has her manuscript.

Kim's bestie says the guy is mistaken though ... she says she was in charge of going through Kim's belongings and distributing them to family and friends after Kim died, and if anyone would have found a manuscript it would have been her ... and there wasn't one.

But, Courtney's lawyer, Ariel Mitchell, tells TMZ ... he obtained the manuscript on a flash drive and it's since been turned over to federal investigators in the Diddy case.