Eric Carle is a well-known illustrator who used collage. Here are several resources for his work:
Watch Eric Carle recreate a Very Hungry Caterpillar:
Or listen to a reading of his the Polar Bear book he illustrated:
Lois Ehlert is another illustrator that many of you are familiar with:
How many of you know Chicka Chicka Boom Boom? She's the illustrator!
Here's a video of someone reading her Painting a Rainbow book:
You can learn more about this author/illustrator in her Scraps book--read aloud here:
Bryan Collier is another great collage artist who combines watercolor with his paper designs.
Learn about Collier and his work here (He talks about The Snowy Day, but he didn't write/illustrate that one!):
Take a peek inside his book Uptown: https://us.macmillan.com/books/9780805073997
Mo Willems took cut paper and collage to 3D in Nanette's Baguette!
See how he created his scene:
Steve Jenkins uses his cut paper work to illustrate books like Actual Size
Here's a read aloud version of his Biggest, Strongest, Fastest book:
Shawn Harris used cut paper / collage to illustrate Dave Eggers' book about the Statue of Liberty: Her Right Foot.
You can see more images of the book through Her Right Foot at Google Books.