Musician Benji Madden "couldn't be more proud" of the contribution his wife Cameron Diaz is making to the world through her latest self-help beauty guide The Longevity Book.

The 43-year-old actress tied the knot with the 36-year-old Good Charlotte rocker in 2015, and as she promotes her new book, Benji took to Instagram on Friday (15Jan16) to lend his support.

Posting an image of the cover for The Longevity Book, which features Cameron's face without make-up, he cooed, "I Couldn't be more proud of my Bad A** wife. She wakes up everyday on a mission to try and make the world a better place.

"I'm always amazed by the courage, strength and vulnerability she shows, relentlessly encouraging other women to love themselves. It has been amazing to get to watch her journey with her New book! #theLongevityBook #April5th @thebodybook @camerondiaz #lovemywife."

Cameron released The Body Book: Feed, Move, Understand and Love Your Amazing Body in 2013 via publisher HarperCollins and the health-related title became a New York Times Bestseller.

The There's Something About Mary star hopes her The Longevity Book will also make a difference in the lives of people who may be struggling with the aging process.

"I am so excited I can barely contain myself!!" she wrote on her own Instagram account on Thursday (14Jan16). "As soon as I saw it myself, I wanted to share with you the cover of THE LONGEVITY BOOK. I am so proud of this book and very much looking forward to engaging us all in a new conversation about aging - how to do it with strength, grace, health and wisdom.

"I also wanted to thank you again for your beautiful submissions, which we'll eventually get to show you as part of the cover beyond the cover; a perfect representation of women standing beside other women, as a united front, to stage, change and incite progress. I can’t wait until hard copies hit the stands in April so we can really get started!!"