I don't think said person, if still in prison, can legally profit from his book sales. But , that doesn't mean he can not profit from it after getting out.
For example, locally here there is a young man, the so-called "Barefoot Bandit", who stole and wrecked some private airplanes as part of his illegal shenanigans. Cost the owners many thousands of dollars. He is writing (ghost wirting) his story. He is legally unable to profit, but the money can be used to pay restitution to his vicitms.
One day he'll get out and then , I suppose he will profit.
In the case of prison workers of all stripes assembling saddles or whatever, yes, I would buy the product. They get paid something like 75 cents an hour. It's nothing, but the skill is necessary. They need something to help them restart their lives once out.
I mostly agree with Tiny, who's said it best.
The saddles which use prison labor - yes, I'd buy. (But they're too expensive for me, LOL.)
The book about a guy's crimes - no, never would I buy.
The book written about another topic by a felon - if I knew the author was a felon and if he was NOT remorseful and trying to make amends for his crimes, then NO I would not buy it. I think that prison sentences mean that the person has suffered the consequences of the crime, imposed by society (law). It does not mean that they have made amends to their victims. In God's eyes, they still have some work to do even if they've paid their debt to society, so to speak. And, as a Christian, I am supposed to forgive him for what he did to me but it doesn't mean that I have to forget about it and certainly doesn't mean that I have to treat him like any upstanding person in society. There are consequences for his past actions - a child abuser doesn't get to babysit; a thief doesn't have access to my belongings; an embezzler can't be a clerk in accounts payable; and a convict can't vote or own a gun. Edited to add - Even while I don't have to trust him, I am supposed to love him in the Christian sense of the word, as he is loved by God, which is hard, too.