Cassie says Diddy is fishing around, demanding she turn over her bank statements and a book she wrote about him ... and she wants a judge to put the kibosh on his legal request.

According to new legal docs, obtained by TMZ, Cassie is trying to get the court to kill a subpoena she claims she was served by Diddy's legal team.

Cassie says Diddy sent her a subpoena March 19 demanding she turn over "all draft memoirs, autobiographies, narratives, diaries, journals or notes ... and any communications about plans to publish or threaten to publish such a document."

What's more, Cassie also claims Diddy demanded she hand over her bank statements.

The subpoena is interesting on a couple levels ... because earlier in Diddy's case one of his attorneys, Marc Agnifilo, claimed Cassie had told Diddy she had written a book about him that he could "catch and kill" for $30 million.

Agnifilo claimed back in September that Cassie had her lawyer call Diddy's lawyer on a recorded line informing him she had written a book about their relationship that she planned to publish. Agnifilo also claimed Cassie indicated to Diddy he could buy the rights to the book for $30 million to prevent it from being published.

Thing is ... Agnifilo claimed Diddy balked at the alleged offer. He claims Cassie switched lawyers and her new attorney told Diddy in November 2023 that the book was no longer important because a civil sex case was going to be filed.

As you know ... Cassie ended up suing Diddy for rape and abuse, and the day after she filed they reached a settlement.