I've read nothing of Amorth's, but have read most of Malachi Martin's books. In that context, I'm quite surprised at Amorth's age as Martin frequently commented on the physical toll most exorcists undergo, and how few of them rarely have a full life expectancy as a result...
I have read some of Martin’s books, but he is an ex-priest and I do take what he says with a grain of salt. Our diocese exorcist has lead a very long life! Fr. Amorth’s books were great reads, I do recommend them.