This book caught my interest because of its "what if" aspect. What if Prince Arthur, the eldest child of Henry VII, had survived to become king after his father? Arthur had been groomed from infancy to be a king and to cement the rule of the Tudor dynasty that his father had worked so hard to establish. His sudden death, at age 15, destroyed all of Henry VII's planning and ultimately brought Arthur's younger brother to the throne as Henry VIII. His kingship followed a very different path from what Arthur's probably would have. As a result, while Arthur's life was too short for him to achieve much of anything, his death dramatically changed the course of English history.
Unfortunately, despite its compelling premise, the book itself simply wasn't all that interesting. I found Cunningham's writing style rather simplistic, with mostly short sentences and long paragraphs that didn't flow all that well. There was also quite a bit of repetition of information, though some of it was presented out of order (people introduced fully only after they'd been mentioned in passing several times, for example the first Marquis of Dorset, whose relationship with Henry VII is discussed several times before Cunningham thinks to mention that Dorset was a half-brother to Henry's queen, Elizabeth of York).
I also found some of Cunningham's discussion of family relationships a bit odd. Early on, he says: "As Earl of Richmond, [Henry VII] was the last nobleman to share the blood of the Lancastrian kings (his father, Edmund, had been a half-brother to Henry VI)." While it is true that Edmund was a half-brother to Henry VI, this was through his mother, Catherine de Valois, the widow of Henry V -- not through the Lancastrian line. Henry Tudor's Lancastrian claim came through his mother, Margaret Beaufort, who did indeed share the blood of all the Lancastrian kings, since both she and they were descended from John of Gaunt.
Later, Cunningham discusses the Yorkist claim to the throne, saying "Richard, Duke of York had inherited the Mortimer estates and the dormant Mortimer claim to the crown through dual descent from Edward III." He notes that Anne Mortimer, sister of the last Mortimer Earl of March, was both a descendant of Edward III and married to another descendant, Richard, Earl of Cambridge. Cunningham says, "It was this dual ancestry to Edward III that led to Roger Mortimer being proclaimed in parliament as Richard II's heir in 1385." The problem here is that the Mortimer claim was not a dual one. The Mortimers were descended from Lionel, Duke of Clarence, Edward III's second surviving son. Richard, Earl of Cambridge, was descended from the fourth son, Edmund, Duke of York. Therefore, it was Richard, Duke of York (son of Anne Mortimer and Richard of Cambridge) and not his Mortimer ancestors who united the the two lines and could claim dual descent from Edward III. Cunningham emphasizes the Mortimer claim (which was the senior one) at various points, but his explanation of the Yorkist ancestry is somewhat confused overall.
On the subject of ancestry, I quibble with Cunningham's calling Elizabeth of York the "last heir" of the Yorkist kings, since although her brothers were presumed dead, she did have several surviving younger sisters who presumably would have been viewed as Yorkist heirs in the event of Elizabeth's death.