When reading the classical children's book series Swallows and Amazons by Arthur Ransome, I started with the eponymous first book Swallows and Amazons, but read Peter Duck before Swallowdale. Thus, I first read Peter Duck in the same spirit as the other stories - the children going on boats and having adventures, albeit rather more thrilling adventures than their usual summer-holiday fare. It was only after reading Swallowdale that I realised Peter Duck was actually metafictional: considered to be a work of fiction even within the world of the Swallows and Amazons.
It was much later that I read Missee Lee, which I assumed to be in the same metafictional category. Although not featuring the character of Peter Duck, it does seem to follow on from the novel Peter Duck, and it similarly stretches the reader's suspension of disbelief much more than most of the books in the series.
Other candidates for possibly being metafictional include Great Northern (which also features the boat Wild Cat) and Pigeon Post (wherein, if I recall correctly, the children successfully dowse and mine copper). But these I'm less sure of.
Which of the books in the Swallows and Amazons series are considered to be metafictional?
This question would probably be best answered using intertextual references (Peter Duck, for instance, is clearly metafictional because of the way it's described in Swallowdale - are there other inter-book references like this?), but I'm open to being convinced by other evidence too.