"Island of Adventure" was the first of Blyton's "Adventure" series books, and was published in 1944. American readers may have encountered it in their childhood libraries as "Mystery Island", the title adopted for the U.S. edition.
The book introduces Jack (the bird lover) and his sister Lucy-Ann (a bit on the timid side). They are orphans who are informally adopted by Mrs. Mannering because of their friendship with Mrs. Mannering's children and because of her great fondness for them. Mrs. Mannering's children are Philip (the animal whisperer) and Dinah (squeamish about animals, but level headed and independent). The cast is rounded out by Kiki, Jack's pet parrot and the comic relief in the books. The children's ages range from 14 for Jack and Philip to 12 for Dinah and 11 for Lucy-Ann.
In each book the two sets of siblings find themselves on school vacation with Mrs. Mannering, (and later also with Bill Smugs - Mrs. Mannering's friend and a member of an unspecified British secret service). Each book starts out with the family going on a simple, relaxing holiday, but mystery, suspense and skullduggery always rears its head about three chapters in. The books feature islands, castles, underground rivers, subterranean tunnels, spies, smugglers, assassins, and other neer do wells.
In "Island of Adventure", the children are staying at a craggy old house set against a steep cliff. From there they can see Gloom Island. After learning to sail they visit the Island and all sorts of mysteries are uncovered and adventures undertaken.
This book sets out all of the characters, their relationships to each other, their various personalities and the group dynamics, (Philip teases Dinah, Lucy-Ann dotes on brother Jack, and so on), that will be consistent throughout the series. It's not necessary to start the series here, or to follow the books chronologically, but it probably would be helpful since this book does establish all of the basics. (Each later book does rather efficiently set this out again, in the first chapters, for new readers.)
This is probably the weakest book from a girl's point of view, since the two girls are much less involved in the most exciting parts than in any other book. This is also the book with the most tone deaf references to ethnic stereotypes, (unless you get a later edition where some of that has been edited out). As I say, the books get more balanced as the series goes on. Perhaps the strongest note, apart from the whiz bang adventure, is that apart from some teasing and the like all of the children trust and admire each other, and the siblings are loyal to each other and their friends. There is none of the incessant sibling conflict that is popular in many current books. There is no moralizing or preaching, but there is a powerful undercurrent of courage, bravery and doing one's duty that is always in the background.
Blyton was criticized by "serious" reviewers for using short declaratory sentences and an undemanding vocabulary. Her response was that she did not listen to any critic who was over 12 years old. That said, these are not light weight or simpleminded books. The characters have charm and individual personalities. There is no pretense of grand literary accomplishment, but these are exhilarating and suspenseful adventure tales, well told. It is a testament to their quality that most of the reviews at this site have been written by people who read the books in the childhoods and still remember them vividly.
And, these books do stand the test of time. Caves, tunnels, floods, deep holes, secret passages, hidden doors, underground rivers, abandoned buildings, creepy noises, mysterious lights and sounds - all of the standards of tween adventure - start here, in the hands of a writer who knew how to tell a gripping story.
These books were hard to find in the U.S. until reprints began to appear. I'm delighted to see that we now have Kindle versions, which makes it easy and relatively inexpensive to try one. If you have a tween reader who likes gung-ho adventure and imaginative larking about, then one of these might be worth a try.