'Curiously enough' is just another way of saying 'Curiously' - you could also say 'strangely'. As regards that phrase: I can only read it, and reread it, trying to make sense of it... It is either taken out of context, or rubbish... OK - so I googled it. It is somewhat meaningless out of context. What the original writer (Nabokov) is saying, is that a 'good reader' has to re-read a 'good book' to fully understand it. The first time you read it, you are just reading the words, and it is only on re-reading it, that you start to understand the deeper meaning.

Because it is a very curious opinion.