I found an article called Why Lila Matters on the American Girl website and it talks about Lila's story, including an interview by Erin Falligant, who wrote Joss's books and Lila's journal and book.
"Our Girl of the Year, Lila, is a confident athlete who has been in gymnastics for years. When she gets a last-minute chance to go to horseback riding camp, she’s excited to try something new but worries she’ll fall behind in gymnastics. Her worries fade when she meets Hollyhock, a young horse at the stable. Training Hollyhock helps Lila learn that friendship can take many forms and that trying two hobbies can help you be better at both."
"When Lila gets a last-minute chance to go to horse camp, she immediately falls in love with Hollyhock, a gold-colored Palomino. Hollyhock is alone in a paddock because she’s new and the other horses haven’t accepted her yet, and Lila can relate. Just like me making new friends at gymnastics, Lila thinks. She decides that just as Coach McKenna believes in her and brings out her strengths, Lila wants to do the same for Hollyhock. By gently grooming her, riding her calmly, and helping Mr. Benson train her so she doesn’t spook as easily, Lila helps Hollyhock shine brightest of all the horses at Honeycrisp Hill."
(Yes. Coach McKenna is McKenna Brooks, Girl of the Year 2012!)
When Lila starts training for the Xcel Gold team, she’s ready to do her best and make Coach McKenna proud. But for the first time ever, she’s not 100 percent confident. The skills are advanced, and she’ll have to make all-new friends at a brand-new gym. When McKenna shares that sometimes you have to go backward to go forward—returning to the last skill you felt confident doing—Lila realizes she’s ready to answer McKenna’s “Ready to work?” prompt with a resounding “Ready to fly!”
When Freya, one of the counselors at the horse camp, finds out that Lila and Emilia are gymnasts, she tells them that will make them good riders. Their strong legs and sense of balance help them with skills like trotting and steering. Soon after, as Lila works to master new gymnastics moves, she remembers something she learned at horse camp: steering a horse works best when visualizing where you want the horse to go. Turns out, it works when trying to land cartwheels on the balance beam, too!