Take a closer look at four stellar picture-book sequels:

• More than four decades ago, children's author Bill Martin Jr. saw an illustration in a magazine and decided he'd like to ask the artist to work with him on a new picture book. The artist was Eric Carle, and the result of their collaboration was "Brown Bear, Brown Bear, What Do You See?"

With its catchy rhyme and spectacular collage/paint illustrations, the book was an immediate hit. Over the next decades, Martin and Carle collaborated on two other "Bear" books: "Polar Bear, Polar Bear, What Do You Hear?" (focused on zoo animals) and "Panda Bear, Panda Bear, What Do You See?" (endangered animals). Overall, the series has sold 11 million copies.

Before he died in 2004, Martin sent Carle the manuscript for one more "Bear" book, focused on native American animals such as red foxes, mountain goats and prairie dogs. The book "Baby Bear, Baby Bear, What Do You See? (Henry Holt, $16.95) has just been published, and it's a winning finale to a popular series.

Like the previous books, "Baby Bear" moves readers quickly through the pages with its cadenced text: "Baby bear/ Baby bear/ what do you see? I see a red fox/ slipping by me" ... and so on until the grande finale that references all of the animals once more. As always, Carle's signature art adds beauty and interest to Martin's text. (Ages 3-7.)

• She's a preschooler with firm opinions, irrepressible energy and a take-no-prisoners approach to life. She's Olivia, the porcine heroine who burst on the scene in 2000 in the picture book created by author-artist Ian Falconer. The book won a Caldecott Honor, and millions of fans were captivated by the feisty Olivia.

Since then, Olivia has starred in three other adventures: "Olivia and the Missing Toy," "Olivia Saves the Circus" and "Olivia Forms a Band." Now, Olivia takes on the holidays in her newest book, "Olivia Helps With Christmas" (Atheneum, $18.99).

As anyone who knows her well could predict, Olivia's "help" isn't always exactly helpful. But her intentions are good as she creates a centerpiece for the dinner table by cutting off the top of the Christmas tree and unsuccessfully tries to untangle the tree lights.

Like the previous "Olivia" books, her Christmas adventure is enlivened by Falconer's trademark illustrations -- mainly done in gray, white and black, with bits of red, green and yellow paint adding color and flair. His deceptively simple line drawings, especially of Olivia, will spark laughter in adults as well as children. (Ages 3-6.)

• Don't tell author Doreen Cronin and illustrator Harry Bliss that bugs don't write diaries. After the success of their book "Diary of a Worm" and its sequel, "Diary of a Spider," they won't believe you.

In fact, they've just published a third volume in the best-selling picture-book series, "Diary of a Fly" (HarperCollins, $15.99). And it may just be the most appealing book of the trio, mainly because of spunky, vivacious Fly, a headbow-wearing heroine who refuses to have her dreams of being a superhero squashed by the reality of her life.

As their previous books, Cronin and Bliss' latest effort mixes deadpan humor with facts about insects. It seems like an impossible combination, but it works like a dream here. Bliss' cartoon-like illustrations, complete with conversation bubbles, add a touch of wild humor to Cronin's lively diary entries. Young readers will particularly find themselves giggling about the contrast between Fly's humanlike behavior in Cronin's text and Fly's behavior as depicted in Bliss' illustrations -- zooming through someone's tomato soup, learning to land on a moving human being, etc. (Ages 4-7.)

• Trixie's had Knuffle Bunny for her entire life, and he's special to her, as the millions of fans of "Knuffle Bunny" know. When Trixie takes him to preschool in "Knuffle Bunny Too" (Hyperion, $16.99), however, she discovers that a girl named Sonja also has a Knuffle Bunny. Inevitably, the two clash over their favorites, beginning with their names ("K-nuffle" vs. "Nuffle"). Their teacher wisely takes away both animals until the end of the school day, and all seems well until the middle of the night, when both girls awake and realize they have the wrong Knuffle Bunny.

Once again, author-illustrator Mo Willems displays his talent for humorously capturing adventures that may seem small to adults but loom large in the lives of preschoolers. Willems' quirky illustrations provide lots of humor as the parents of Trixie and her friend hurry to set things right in the middle of the night. It may not be realistic but, in Willems' capable hands, it sure is funny. (Ages 3-6.)