Melissa Rivers ... Lawyered Up and Ready to Sue Over Joan's Death

Joan Rivers' daughter Melissa has retained a law firm that will file a major lawsuit over her mom's death ... TMZ has confirmed.

We're told the firm -- Gair, Gair, Conason, Steigman, Mackauf, Bloom & Rubinowitz -- will file a medical malpractice and wrongful death lawsuit against the clinic where Joan stopped breathing and the doctors who were involved.

Our sources say Melissa retained the firm Tuesday, but the suit won't be filed for around a month.

We're told letters are going out today to the various people involved in the fatal medical procedure, telling them NOT to destroy records involving Joan's treatment. The lawyers also want phone logs and other documents.

As we reported ... one of the driving forces for Melissa is to figure out how her mom died, but in the end, it's a lawsuit over money.