It felt that by the time I got to this one Blyton's books had become a little more action packed, and the the world of the Famous Five had become somewhat deeper. This one even brings Ragamuffin Jo back into the adventure, who plays an important role, as well as bringing a new person over from America to hide from potential kidnappers. We even have a much broader adventure where we have scenes on Kirrin Island, in a fair ground, and even in a mysterious house. Also while we do have the typical crooks in this story, they seem to lurk in the background and we don't really meet them until the end.
Once again Uncle Quentin is working on a secret project with an American scientist, but there are some nefarious people (most likely Russians, considering the time it was written, though Blyton never actually implicates nationalities in her books) who want the secrets for their own end, so they attempt to make plans to kidnap the American's daughter. The problem for Quentin is that the American loves his daughter so much that if he is kidnapped then he will spill the beans (and Quentin makes such a song and dance, much to Fanny's horror, about how he wouldn't do that if George gets kidnapped). As such they decide to bring her over to Kirrin Cottage and not only hide her here, but disguise her as a boy (much to her horror).
The series by this time is coming somewhat more mature, though as many have suggested, it seems like they exist in this ageless world of endless holidays. None of the children grow up, and even if they are teenagers, George is still at an age where she can be mistaken by a boy. There will come a time when George will find it much more difficult to pretend to be a boy (especially during puberty) however there are suggestions here that despite her desire to be a boy, it is difficult to her to actually disguise herself as one. In a way they point to the hair and suggest that despite it being short it still has a feminine quality about it (and I wonder if there are some underlying currents, though still, this is a kid's book).
There are 21 Famous Five books and 15 Secret Seven books, though I guess the difference is that where the Famous Five books unfold over a period of a week, the Secret Seven unfold over much shorter time frames. Also, the need for the holidays is evident because they are off at boarding schools while the Secret Seven live within walking distance of each other (though we are never told which schools they go to). I sort of wonder why they never meet up, but I suspect that it is because the world of the child is much smaller than, say, the world of Batman and Wonderwoman.