Good Old Timmy is a short story by Enid Blyton following the further adventures of the The Famous Five. It was originally published in 1956 in Enid Blyton's Magazine Annual #3; as this was the same year as Five on a Secret Trail (and assuming that this is an indicator of internal chronology) this story might be considered #15.5 in the series.
This is a rather disappointing story. It is short, hurried, and bland. It has some inelegant expository lumps, and while The Famous Five are not known for unique plots and clever twists, this is much more cliched than most of their stories. It also shares the basic plot of the 1955 book Five Have Plenty of Fun, which is a much better read.
Minor spoilers: While on summer holidays at Kirrin Cottage, the famous five meet a young boy who turns out to be the son of one of George's father's scientist friends. Shortly after meeting him, he is kidnapped off the beach in broad daylight, and the Five, with help from George's dog Timmy, manage to track him down and save him.
Coming in at under 20 pages (but still containing seven 'chapters'), this is a quick and very easy read. Even so, I don't particularly recommend it. However, since it is currently only available in various collections of Famous Five stories, and you will only acquire a collection if you are a fairly obsessive fan, you will read it if you have a chance. As least you've been forewarned.
Good Old Timmy is present in most (perhaps all?) Famous Five short story collections, including Good Old Timmy and Other Stories, Five Have a Puzzling Time and Other Stories, and The Famous Five Short Story Collection. Against all reason, it is also available as a stand-alone 'book'.