," which is due to be published today.
She visited Watts in prison three times for research and they wrote to each other.
In these letters, Watts admitted that he unsuccessfully tried to smother their young daughters after an argument with Shannan, using "a pillow from their bed."
He wrote that the little girls both regained consciousness and got out of bed bruised and traumatised after he had killed Shannan, who was 15 weeks pregnant.
Watts said he knew he would kill them on August 12 but "did nothing to stop it" and felt "mad" when he discovered they were still alive after his murder attempt the next day.
HOW YOU CAN GET HELP
Women's Aid has this advice for victims and their families:
Always keep your phone nearby.
Get in touch with charities for help, including the Women’s Aid live chat helpline and services such as SupportLine.
If you are in danger, call 999.
Familiarise yourself with the Silent Solution, reporting abuse without speaking down the phone, instead dialing “55”.
Always keep some money on you, including change for a pay phone or bus fare.
If you suspect your partner is about to attack you, try to go to a lower-risk area of the house – for example, where there is a way out and access to a telephone.
Avoid the kitchen and garage, where there are likely to be knives or other weapons.
Avoid rooms where you might become trapped, such as the bathroom, or where you might be shut into a cupboard or other small space.
If you are a victim of domestic abuse, SupportLine is open Tuesday, Wednesday and Thursday from 6pm to 8pm on 01708 765200.
The charity’s email support service is open weekdays and weekends during the crisis – messageinfo@supportline.org.uk.
Women’s Aid provides a live chat service available. from 10am to noon.
You can also call the freephone 24-hour National Domestic Abuse Helpline on 0808 2000 247.
The terrified children found Watts wrapping their mum in a bed sheet, the letter said.
Watts said he told them Shannan wasn't feeling well before dragging her body down the stairs and into his truck.
He then drove his dead wife and two frightened daughters to a remote oil field owned by his former employer, Anadarko, bringing his lunch, a shovel, and rake, the Mail reported.
After dumping his wife's remains on the ground, Watts smothered Celeste while her sister watched before dropping her body through an eight-inch hatch in one of the oil tankers.
Watts told Cadle that "little quiet Bella had a will to live" and was the only one to put up a fight against him, saying "daddy, no!" as he killed her.
He insisted both of the girls were dead when he dropped them in the oil tankers.
Shannan's cause of death was strangulation while an autopsy found crude oil in Bella and Celeste's throats, stomach and lungs.
Watts previously told investigators that Shannan's killing was a spur-of-the-moment crime and didn't admit to planning it.
Initially, he told cops that he had flown into a rage and killed Shannan after falsely claiming she had strangled their daughters.
The letter is the first time he's acknowledged killing his kids.
Watts later pleaded guilty to all three killings and his mistress Kessinger has been taken into witness protection in another state.
Moment Chris Watts confesses to his dad that he murdered his pregnant wife Shannan, saying she had harmed their daughters