Despite the title, this book is more about Karo's mystical adventures than it is about his contribution to Jewish law. In particular, it discusses Maggid Mesharim (MM)- a book allegedly written by Karo discussing his mystical adventures. MM is a diary of Karo's messages from a "maggid"- some sort of holy spirit that spoke through him. (The author assumes for the purposes of his discussion that Karo actually wrote it, though of course there is no definitive way to prove or disprove the claim).
The author thinks the maggid was "an involuntary movement and irresistible automatism of the larynx and lips" that appear to have arisen from Karo's subconscious. This sort of "maggidism" was not unique to Karo; other 16th-century Jewish writers discussed the concept.
Karo's revelations were nothing like prophecy in the Bible; they rarely discussed anything that the average Jew would have benefitted from reading. Many of the maggid's remarks addressed Karo's personal affairs- for example, making predictions about him and his family, and rebuking him for eating and drinking too much. (Evidently, the maggid wanted Karo to be very, very ascetic). The maggid's revelations rarely involved Jewish law; it does not appear that the maggid suggested any conclusions that Karo would not have reached on his own.