Prince William will one day be king, but it is his wife, Catherine, Duchess of Cambridge, who has become his closest confidante and most trusted advisor, says royal family expert Robert Jobson.

Not to be forgotten, in addition to the Platinum Jubilee milestone celebrations marking the Queen's historic 70th year on the throne, another royal milestone is coming in June: William's 40th birthday on June 21.

To mark the occasion, longtime royal reporter Jobson published William at 40: The Making of a Modern Monarch (opens in new tab) on June 9 in the UK and July 25 in the US. In the book, Jobson praised Kate's positive influence, writing, "In the midst of crisis upon crisis, a smiling Catherine never missed a step."

Since meeting William as a freshman at the University of St. Andrews in Scotland in 2001, she was initially his friend, roommate, and girlfriend until they became engaged in 2010 and married in 2011. At every step of the way," he says, "Catherine has always put duty first. And beneath her soft smile is "a modern woman who is like a tiger, passionate about her husband, and thoroughly committed to fighting for her family."

In addition to her own growth, Jobson says, Kate has made William a better man.

"With Lady Catherine by his side, the prince seems more confident, and their chemistry is clear for all to see," he says.

This past weekend, Kate and William were seen constantly together on the "Trooping the Colour" balcony, at a service of thanksgiving to the Queen at St. Paul's Cathedral, visiting Cardiff Castle in Wales, and at a platinum party at the palace.

"She is not only William's most trusted advisor," Jobson wrote, "but someone who puts family and duty above all else.