Well, one could argue there were 40 days or so between the resurrection and the ascension, and that Aslan's leaving at the end was like the ascension - since IIRC, he never showed up in Narnia proper after that, not like he did before his death... No wait I just remembered he totally shows up in Prince Caspian so nevermind that train of thought.

With me, the last time I read Narnia was shortly before the first time I read Lord of the Rings. As soon as I realised how much richer symbolism is than allegory, I didn't ever feel the need to read Narnia again.