If by "the author" you mean Geoffrey Drumm, I have a copy of his book and I can tell you that his work doesn't talk about the conversion of basalt via hydrochloric and sulfuric acid into magnesium sulfate. His book is a fictitious narrative that bakes in his research. I'm reminded often of the words of Evey Hammond's father from V for Vendetta who is said to have said that "artists use lies to tell the truth". Drumm moved to Egypt a few years back to more fully immerse himself in his research and Egyptians often don't take kindly to suggestions that the pyramids were used for anything other than Pharaonic burial so by publishing his book as a fictitious narrative, he is leaving himself an easy "out" if anyone comes at him with anger over his research. Drumm suggests that the Great Pyramid was used for sulfuric acid production and that the Central Pyramid was used for hydrochloric acid production. In his youtube channel content (The Land of Chem), Drumm does offer some of his own speculations as to what the ancient Egyptians were using these synthesized chemicals for, he doesn't push the idea that they were creating what we would commonly refer to today as Epsom salts. That's my speculation which stands upon the shoulders of his work. I've tried to reach out to Drumm before, multiple times, but I'm sure his inbox is inundated by lots of people with lots of theories and it's easy for my attempts to communicate with him to get lost in the crowd, so to speak. Maybe some day I can get him on a call and explain how beautifully my research dovetails with his and get him to dedicate some of his time and attention towards looking for evidence of magnesium sulfate production.