A US writer who penned a tract entitled "How to Murder Your Husband" was sentenced to life in prison on Monday for fatally shooting her spouse.

Nancy Crampton Brophy, 71, will be eligible to apply for parole in 25 years, a judge in the northwestern state of Oregon told her.

Crampton Brophy's month-long murder trial heard how the author had used a gun barrel bought on eBay to kill her husband in the hope of collecting hundreds of thousands of dollars in life insurance.

The weapon, which she claimed had been acquired as research for a new novel, has never been found.

Chef Daniel Brophy was found on the floor of a classroom in a now-defunct culinary institute where he worked, in June 2018. He had been shot twice.

CCTV footage showed his wife driving in the area at the time.

The author, whose "Wrong Never Felt So Right" series of novels include "The Wrong Husband" and "The Wrong Lover," claimed she had no memory of being there, but surmised that she would have been in the neighborhood trying to get inspiration for a new work of fiction.

Crampton Brophy, whose lawyers said last month they would appeal the verdict, denied the murder, insisting that years of financial struggle were behind the couple and that she had no reason to kill her husband.

"Where is the motivation? I would ask you," Crampton Brophy said as she sparred with the prosecutor during her trial.

"An editor would laugh and say, 'I think you need to work harder on this story, you have a big hole in it.'"

Her blog post "How to kill your husband," which is still readily available online, discusses methods and motivations for dispatching an unwanted spouse.

These include financial gain and the use of a firearm, although it notes guns are "loud, messy, require some skill."

"But the thing I know about murder is that every one of us have it in him/her when pushed far enough," the essay says.