I just did a quick search in the Amazon books section which did produce a bunch of Cubase books. However they mostly seemed to be older and about earlier versions of Cubase. Which would imply there isn’t much of a market anymore for Cubase Books. That said a book about Cubase 6 etc. could still be useful. For the most part the way Cubase works hasn’t changed much in recent versions. Granted these books wouldn’t include newer features & capabilities, but what they do cover will in general still be valid. And where it isn’t correct, it should be fairly clear what to ignore comparing the book to what you see on your screen. Also the “what’s new” videos can help bridge that gap.
Bad news is it doesn’t seem like Amazon has anything focused on C8, and with C9 just around the corner everything will soon move another generation back.