Bertie at Bedtime by Marcus Pfister

From chapters.ca:
The acclaimed creator of the Rainbow Fish series returns with Bertie, an irrepressible hippo who's not quite ready for bed. Bertie might wear his daddy out before he's actually ready for sleep.

I Don’t Want to be a Pea! By Ann Bonwill and Simon Rickerty

From chapters.ca:
Hugo is in no doubt as to what he and Bella should go as to the Hippo- Bird Fancy Dress Party. They will go as the princess and the pea and he will be the princess (of course!) leaving poor Bella to be the pea. Indignant, Bella refuses. She has a better suggestion - they will go as a mermaid on a rock and she will be the mermaid (of course!) leaving a huffy Hugo to be the rock. Hugo is not happy. But neither is Bella. They just can't decide on a costume and things reach a crisis when they both flounce off saying they don't want to go to the party at all. After time to consider, both Hugo and Bella calm down and (privately) both choose to dress as a pea as a gesture of making it all up to each other. So when they both arrive at the party as peas, Hugo and Bella announce proudly that they are "two peas in a pod, just as it should be" underlining the message of the book that friendship is all about give and take. With wonderfully absurd illustrations from new talent Simon Rickerty and a great read- aloud text, this is a made- for- sharing picture book.

Patricia von Pleasantsquirrel by James Proimos

From chapters.ca:
First things first, Patricia von Pleasantsquirrel is not a squirrel. She's a girl - a princess, in fact, only she hasn't found her princessdom. But that's about to change. Inspired by a book she reads, Patricia sets off for a place where she can stay up till midnight, eat cake before dinner, and have not just a dog but a great white stallion. She finds it. And a whole lot more than she bargained for. James Proimos pays cheeky homage to a classic picture book favorite in this disarming, laugh- out- loud funny tale of a prideful little girl, her royal goldfish, her airplane, and her cadre of hippos. Two levels of comedy and plenty of visual gags make this a great read for kids and parents alike.

A Mama for Owen by Marian Dane Bauer, illustrated by John Butler

From chapters.ca:
Owen the baby hippo and his mama were best friends. They loved to play hide- and- seek on the banks of the Sabaki River in Africa. That was all before the tsunami came and washed Owen's world away. But after the rain stops, Owen befriends Mzee, a grayish brown tortoise. He plays with him, snuggles with him, and decides he just might turn out to be his best friend and a brand- new mama. Inspired by the tsunami of 2004, acclaimed storyteller Marion Dane Bauer and celebrated illustrator John Butler depict this heartwarming true tale of healing, adoption, and rebirth - with splendid illustrations and oodles of love.

Veronica on Petunia's Farm by Roger Duvoisin

From chapters.ca:
Veronica, the very conspicuous hippopotamus, is happy to move to Mr. Pumpkin’s farm. There will be so many animals to gossip with! But the other animals do not return Veronica’s hearty "Good morning!" And they say nasty things about her. Her feelings hurt, Veronica hides in the barn for a whole week — the animals spy on her as she wastes away and looks morose. Feeling guilty, the animals offer gifts to cheer her up, and when she finally joins them in the barnyard, they see the biggest smile ever on Veronica’s face.

Hippo Goes Bananas! by Marjorie Dennis Murray, illustrated by Kevin O'Malley

Hippo has a tooth ache and, when the other animals see her, they think something else is going on.

Hippos go Berserk! by Sandra Boynton

From chapters.ca:
This offering from the inimitable Sandra Boynton is a wonderful counting lesson embellished with intrinsically funny hippos. One hippo is all alone, so he calls up two others, who come over...followed by three hippos, then four more, and so on until there's a party of hippopotamic proportions in the living room! But when the party's over, the hippos depart in reverse order, a group of nine first (plus one unexplained beast guest), then eight, seven, and so on until the first hippo is alone again and missing his 44 friends. The jaunty rhyming text sets up some great visual gags. This is a truly fun and inventive book.

The Blue Hippopotamus by Phoebe Gilman, illustrated by Joanne Fitzerald

From chapters.ca:
A beautiful and unusual story about the power of love and generosity. Reminiscent of classics such as The Velveteen Rabbit, this is the book Phoebe Gilman was working on when she died in 2002. Renowned illustrator Joanne Fitzgerald completed the project with illustrations inspired by Phoebe Gilman's sketches which were based on The Blue Faience Hippopotomus, an original story by Joan Grant. A young hippopotamus is so enamoured with a beautiful Egyptian princess that he begs a magician to turn him into something she could love. Although he doesn't hold the power to turn the hippo into a human, the magician grants one wish and also transforms him into a blue ceramic hippo which becomes adored by the young princess and is carried with her everywhere.

Owen and Mzee by Isabella Hatkoff, Craig Hatkoff, and Dr. Paula Kahumbu, photos by Peter Greste

From the back of the book:
This is the true story of two great friends: a baby hippo named Owen and a 130-year-old giant tortoise named Mzee.
In December 2004...a frightened young hippo, separated from his family by the devastating tsunami in Southeast Asia, adopted an ancient Aldabra tortoise as his "mother." And the old tortoise, for years a longer, accepted the baby hippo as his own. They are now inseparable. This wholly unexpected friendship captured hearts everywhere.

These two picture books are available as simpler board books for younger readers. There is also a TV documentary about the incredible friendship between these two animals.

George and Martha by James Marshall

These stories about two hippopotamus friends, George and Martha, have become a classic of children's literature.