Charles Tang

At an old library, the Alden children discover a boy who needs their help and a stranger who is after a valuable object they found in the library.

When their favorite pizza restaurant almost goes out of business, the Alden children try to help their friends stay open while they find out who is behind all their problems.

The Aldens try to help a visiting Russian gymnast who is surrounded by mysterious people and events and who seems to have a secret.

The Aldens participate in a three-day bicycle ride and soon find that someone seems to be intent on keeping others from completing the event.

While on a cruise with their grandfather, the Alden children help discover who is behind the mysterious troubles of the ship--disconnected phone lines, engine trouble, a man overboard--and still have fun sightseeing, eating, and swimming.

While visiting a Navaho Indian reservation in New Mexico, the Aldens try to save a forest under threat of development by proving it is the site of a lost, buried village, but someone is sabotaging their dig.

While helping fix up the old house their cousins have bought, the Aldens hear stories about its being haunted and find evidence suggesting that there may be a ghost there.

On a camping trip to Arizona with their grandfather, the Alden children search for a lost gold mine.

When they help out at a guide dog training school, the Alden children meet several people who seem obsessed with a particular dog named Ginger.

While visiting Lake Lucille in the Adirondack Mountains, the Aldens hear stories about a monster in the lake and then encounter evidence that it may be real.