Lyddie and Jip by Paterson
Speak and Catalyst by Anderson
The Giver and Gathering Blue and Messenger by Lowry
Walk Two Moons, Absolutely Normal Chaos and Chasing Redbird by Sharon Creech
Killer's Cousin and Locked Inside, Nancy Werlin
Scarlet from Sarah Dessen's
Someone Like You pops up in Dessen's This Lullaby.
Angela Johnson's new one First Part Last links to Heaven.
Most of Cynthia Voigt's books has reoccuring characters.
Forged by Fire, and Tears of a Tiger by Sharon Draper
Ponyboy Curtis (from The Outsiders) reappears in Rumble Fish some of the same characters reappear in That Was Then This Was Now.by S.E. Hinton
Athletic Shorts has many characters
in the short stories that were previously in novels. by Chris Crutcher
Whale Talk and Staying Fat for Sarah Byrnes have teachers who were characters in previous novels - Sarah Byrnes links with Stotan,
IRONMAN links with STOTAN, by Chris Crutcher
The Boy Next Door and Boy Meets Girl, by Meg Cabot have characters appearing in both books.
Madeline L'Engle's books feature characters crisscrossing paths. If I remember right, Zachary Gray appears in both Murry stories and the Austin stories.
The Year Of Secret Assignments makes brief character references to her previous novel FEELING SORRY FOR CELIA.by Jacyln Moriarty
Sarah Dessen: many of her books take place in the same town so characters are referred to but are not critical to the plot.
Norma Johnston, Her historical fiction books contain characters who are mentioned in other books or are children/grandchildren of people in other books
The pianist Mr. Felix from Rumer Godden's wonderful ballet novel THURSDAY'S CHILDREN made a much earlier appearance in her A CANDLE FOR ST. JUDE.
Charles de Lint's short stories and novels almost always feature characters that have appeared in other novels and short stories. It's a lot of fun because often a character who is mentioned in passing in one story is the protagonist of another. Most of his stuff has been published with an adult audience in mind, but I believe _Waifs and Strays_, a story collection, was published as a YA and collects stories from his other collections.
Contents Under Pressure, and Bringing Up The Bones.by Lara M. Zeises.
Cynthia Voight titles, and the Moriarty books--Feeling Sorry for Celia and the Year of Secret Assignments.
Sloppy Firsts and Second Helpings by Megan Mccafferty
Truth Or Dairy and Wurst Case Scenario by Clark
Mildred Taylor books