A couple of things I remember from the book, and from my visit to France. Going from memory here, so please forgive me, if I get a detail wrong.
Mummification requires dry heat. The Egyptian mummies had the internal organs removed, and the cavity filled with hot sand.
Incorruptibles, because many were living in monasteries, which were built on less than desirable land, were often buried along side swamps and marshes. Often, the clothing the person was buried in had decayed, but the body had not.
Mummified bodies are stiff and leathery.
Incorruptibles are soft and and supple. I believe it was St. John of the Cross who bled, when they attempted to cut a finger off to be used as a 1st class relic.
Incorruptible bodies often give off a scent of perfume, or other sweet fragrance.
The body of some incorruptibles, have been known to change position.
St. Catherine of Labore, was buried with her eyes closed and her hands laying on her chest, pointing toward her head,as in prayer. She was discovered to be incorrupt in 1830 at which time, they moved her body to the chapel. About the time of the French Revolution In mid 1800’s, there as a riot breaking out in the street outside of the shrine. People inside the shrine-chapel noticed that St. Catherine’s eyes had opened, and her hands were now point toward the ceiling. The excitement flowed into the street and the curiosity of the crowd to see the event, stopped the riot.
BTW, her eyes are blue. I got to see her, and she like St. Bernadette, is beautiful.