Jean Hornsby''s ''Tree-Riffic!''..." is set on ''Shawnee Mountain'' in Pennsylvania in 1947. The Ned and Nora Hammermill family is made up of Jean, Susie, Johnny, Carol and baby, Janice. Jean, at 10, is the oldest and the wisest and central to our story, while the rest of the kids play equally important supporting roles, as ''Johnny'' plays pirate in his treehouse and Susie makes hot steaming biscuits with her mother and Carol looks for her kittens, while Janice steals everyone''s attention as ''the apple in everyone''s eye.'' There is ''Lucky,'' the smartest German shepherd since ''Rin-Tin-Tin.'' And, "Gunner," the retired military horse, who now pulls Ned''s plow. Together, this group of interesting characters take us down the road to a past more innocent and fun, than anything we''ve seen since "The Waltons," made television history. ....Jean loves her black stallion, "Buck," who stumbles and breaks his leg. Johnny plays his trombone to the light of the full moon. Jean sees a beautiful ghost in her mother''s rock garden, where angels also visit and offer their blessings. Ned survives the tornadoes that strike the mountain. This "novel for children," with beautiful color illustrations by John E. Ayers. is told through the eyes of "Mr. Oak" and "Miss Maple," two leaves Jean enters into a 4-H contest in hopes of "winning first prize