Barbie: The Secret Hearts is a short picture book for Barbie fans; I believe the version I read was ever-so-slightly abridged for its inclusion in the Barbie Loves Her Friends picture book, so it may have some slight differences from the standalone version.
The story is quite simple. Barbie receives a note from a secret admirer, who proceeds to send her on a flirty scavenger hunt through the pinkest stores in town. The "secret admirer" turns out to be--brace yourself for a shocker--Ken, who asks Barbie to be his date for the Sweetheart Dance (at their high school, I guess, though they look and act far too mature to be teenagers). There's really nothing interesting happening here, but I suppose it's enough to entertain a toddler.
If you have a Barbie-obsessed preschooler in your life, they might get a kick out of Barbie: Secret Hearts. For older fans of Barbie, though, I'd definitely advise skipping it--it's too much money for too few pages, as far as I'm concerned.