Quesrion about the use of そう to mean “it seems”

Hi,
I had to translate this sentence:
難しそうな本を読めた
that should mean "I read a book that seemed difficult"

It's ok and clear for me, only I don't understand the な after 難しそう. I don't think that is related to the adjective, because (if I'm not wrong) 難しい is an adjective in い and not in な, so does someone know what's the meaning of that な?
Thanks!