The Young Adventurers

The books included in the series were originally written as stand alone books or short stories, but they were revised and heavily updated by Enid Blyton's daughter, Gillian Baverstock, so that they featured consistent characters throughout and could be published as a series.[2][3] The series was first published posthumously as the Riddle Series in 1997 by Collins. It was republished as The Young Adventurers Series in 2004 by Award. The Young Adventurers are illustrated by Patricia Ludlow.

The Young Adventurers:

1. The Young Adventurers at Holiday House[4]
2. The Young Adventurers and the Mystery That Never Was[5]
3. The Young Adventurers and the Rajah's Ruby[6]
4. The Young Adventurers and the Hollow Tree[7]
5. The Young Adventurers and the Hidden Treasure[8]
6. The Young Adventurers and the Boy Next Door[9]

The Riddle Series:

1. The Riddle of Holiday House[10] ISBN 1-84135737-5
2. The Riddle That Never Was[11] ISBN 0-00-694574-0
3. The Riddle of The Rajah's Ruby[12] ISBN 0-00-694575-9
4. The Riddle of The Hollow Tree[13] ISBN 0-00-694576-7
5. The Riddle of The Hidden Treasure[14] ISBN 0-00-694577-5
6. The Riddle of The Boy Next Door[15] ISBN 0-00-694578-3

The original books were:

1. Holiday House[16] (1955): Characters include twins Pat (Patrick) and Mary. Characters at the holiday house include Mrs. Holly, Ruth, Maureen, Tessa, Graham, and John.
2. The Mystery that Never Was[17] (1961): Characters include Nicky Fraser, Nicky's dog Punch, Nicky's maternal uncle Bob, Nicky's best friend Ken, Ken's sister Penny, and Penny's friend Winnie.
3. Adventure of the Strange Ruby[18] (1960): Characters include Pat and his sister Tessa, twins Faith and David Gatherhood, and Faith and David's governess Miss Lawley.
4. Hollow Tree House[19] (1945): Characters include siblings Peter and Susan, their aunt Margaret, their uncle Charlie, their friend Angela, and Angela's dog Barker. *Hollow Tree House[20] was also republished in 1992 by Dean, as book 100 in the Dean's Reward Series.
5. The Treasure Hunters[21] (1940): Characters include siblings Jeffrey, Susan, and John, their mother, their grandparents (the Greylings), and their dog, Rags as well as Mr. Potts and Farmer Tibbles.
6. The Boy Next Door[22] (1944): Characters include siblings Robin and Betty, their friend Lucy, Lucy's dog Sandy, their neighbour Kit Anthony Armstrong, and his guardian, known as "the Dragon".

In this series, siblings Nick and Katie, along with various friends solve different mysteries. In the book The Riddle of The Hollow Tree, it is told that Nick and Katie's parents have died, and they are living with their aunt and uncle. At the end of the events in Hollow Tree, Nick and Katie have been adopted by their friend Laura's family, and Laura joins them in the remaining books in the series.

1. The Holiday House: Nick (Pat) and Katie (Mary) are sent to stay at Holiday House as they recover from illnesses. While at the house they see a light shining from an abandoned house and discover that there are secret passages from a cave and within Holiday House. They also solve the mystery of thefts occurring at the Holiday House.
2. The Mystery That Never Was: Nick and Katie's private investigator Uncle Bob comes for a rest and to recover from illness. He seems bored, so Nick and his friend Mike invent a mystery for him by writing a fake, coded note. They set the mystery on Skylark Hill, where there is a burned out house formerly owned by an Eastern prince. However, the mystery begins to come true, and it requires Nick and Katie, their friends Mike and Penny, Uncle Bob, and their dog Punch to solve it. They discover that there are foreign smugglers operating in the tower. In the original book, Uncle Bob is Nicky Fraser's uncle, Ken is Nicky's best friend, Penny is Ken's sister, and Winnie is Penny's best friend.
3. The Rajah's Ruby: Nick (Pat) and Katie (Tessa) read the newspaper and find out that their friends David and Sophie (Faith) have inherited a ruby. The ruby had been stolen from an Indian temple during a civil war and was believed to be bad luck. The ruby is then stolen and David and Sophie are kidnapped, but they manage to get a letter to Nick and Katie revealing where they are being taken. Nick and Katie manage to cycle from where they are spending their holiday to where David and Sophie are being held prisoner in Brinkin Towers. After their Aunt Dora breaks her leg, and Nick and Katie's mom returns to take care of her, Nick and Katie camp out on an island belonging to Brinkin towers. The twins manage to toss out the key locking them in, and Nick and Katie are able to release them. After their escape they hear the captors planning to move the captives to the island. One of the captors sneaks off and leaves the ruby on the island. He is accidentally scared off the island, and the remaining captors come to search for him. The children hide, but while the captors are on the island they sneak off and take their boat. They alert a policeman, who rounds up the captors and finds the ruby. The ruby is returned to a bank and the children spend the rest of their holidays together.
4. The Hollow Tree: Nick (Peter) and Katie (Susan) are sent to live with their cruel Aunt Margaret and her passive husband after the death of their parents. They make friends with neighbour Laura (Angela) and her dog Russet (Barker). While exploring the nearby woods, they discover a hollow tree, which they begin furnishing with their own items. After they get in trouble for the missing items, and their uncle walks out, they are to be sent to a children's home. Instead they run away to live in the hollow tree house. Angela helps them hide away from the search party that looks for them. One day, Angela comes looking for the tree house, but gets lost due to the guiding string being broken. She gets caught out in the storm, and is found by a rescue party, but becomes very ill from exposure, and keeps asking for her dog. Katie and Nick manage to return Russet to Laura, and end up being adopted by Laura's family.
5. The Hidden Treasure: Nick, Katie, and Laura (Jeffrey, Susan, and John), along with their dog Russet (Rags) go to visit Laura's grandparents at the Greyling Manor. Unfortunately this will be the last time they can visit, as the family estate must be sold to Mr. Potts. Their grandmother tells them that they are out of money, and tells the story of the Greyling's treasure, which had been given to Hugh Greyling by an Indian prince 250 years before, but had been lost. The children discover a cottage in the woods that is overgrown, and clear it out, but when they light a fire they discover that the chimney is blocked. It is found that there is a box containing a map to the treasure blocking the chimney. Mr. Potts manages to see and trace half of the map, and later searches their house secretly for the other half but is unable to find it. The children trick Mr. Potts with a fake half of the map while they decipher the real thing and find out where the treasure is hidden. They end up finding the treasure, running from Mr. Potts, hiding in a farm house, and ultimately returning with the treasure to their grandparents so that the manor need not be sold.
6. The Boy Next Door: Nick (Robin), Katie (Betty), and Laura (Lucy), along with Russet (Sandy) notice that a boy has moved in next door. However, when they ask about him they are told there is no boy living there. Still, they manage to communicate with the boy, and find out that his name is Kit Anthony Armstrong, he is an American boy, and he is in hiding from his uncle. They sneak out together on occasion and at one point find an abandoned houseboat named The Black Swan which they fix up. They get in trouble for cleaning it up, and must hide the existence of Kit. At one point, Kit hides away on the houseboat, but it ends up being where Kit's enemies were planning to hide him anyway. In the end Kit is rescued, and reunited with his father who has also been searching for him.