The Duke of Cambridge made the affectionate declarations in the preface to Elizabeth II: The Steadfast, a biography by former Home and Foreign Secretary Lord Hurd that recounts the reign, character and family life of the monarch.
Prince William has praised his grandmother's sense of humour
In the preface, the father-of-two also reflects on his grandmother's commitment to a life of public service.
"After almost ninety years, we find ourselves in a world that has changed dramatically, almost beyond recognition to the world that The Queen was born into, but where the role of charity, family, duty and compassion perseveres," he writes.
"I think I speak for my generation when I say that the example and continuity provided by The Queen is not only very rare among leaders but a great source of pride and reassurance."
The Duke of Cambridge has written the preface to Elizabeth II: The Steadfast, a biography by former Home and Foreign Secretary Lord Hurd
Prince Charles' son also pays tribute to the Queen's "kindness, sense of humour and her love of family".
"From a personal point of view, I am privileged to witness the private side of The Queen, as a grandmother and great-grandmother."
The Queen’s kindness and sense of humour, her innate sense of calm and perspective, and her love of family and home are all attributes I experience first-hand."
Reflecting on the role of his grandfather, Prince Philip, he says: "I should add that no mention of The Queen is complete without paying tribute to my grandfather Prince Philip, who has devoted his life to supporting her".
The book, which will be published on 27 August 2015, forms part of series of short histories of Britain’s sovereigns entitled Penguin Monarchs, published by Penguin Books.