The books, which are intended for a primarily pre-teen, female audience, explore the characters' adventures in horse racing, eventing, and steeplechase.

Though Wonder is very weak and nearly dies multiple times, through Ashleigh's care, the filly grows up to be a Kentucky Derby and Belmont-winning racehorse.

After book 23, Cindy's Honor, the series skipped forward ten years and began anew with a focus on eventing.

The new series involved Ashleigh Griffen's daughter, Christina, who traded Wonder's latest foal, Wonder's Legacy, for a racehorse, named Sterling Dream, which she turned into an eventer.

Allie was both interested in eventing and racing, bringing together the two disciplines in the Thoroughbred series, although she ultimately decided upon becoming a jockey.

After Karen Bentley left the series, there was a rotating "panel" of contributing authors: Allison Estes, Alice Leonhardt, Dale Blackwell Gasque, Lois Symanski, Mary Newhall Anderson, Karle Dickerson, and Jennifer Chu.

One review noted that while the Thoroughbred books were reliant on the cliché of a bond between a girl and a horse, they were "better written and more knowledgeable" on racing.

[3] Later books in the series were less well-received, with critics noting a significant decrease in quality once Campbell retired from the Thoroughbred novels.