"His books are very interesting."
Translation:Seine Bücher sind sehr interessant.
That could be correct in the right context and with the right tone of voice, but "ganz" is a bit of a tricky one. More often than not, it means "rather" or "quite" (similar to "ziemlich"), while it less often means "very" or "really".
Yepp, What ikwilvertalen said is true. I have to admit, that I'd always prefer ziemlich or ganz and would never use sehr if the author has not won a Pulitzer or a Nobel Price. Maybe it has to do with the region I was born and raised, but the use of very seems to me unnecessary. It's too exaggerated, too flamboyant, too exuberant (I'm exaggerating a bit). A typical dialogue in a restaurant:
Waiter: How was the food?
Customer: War ganz gut. [or even: War nicht schlecht.]
Waiter: That's nice to hear.
What the customer (if he was an American) really meant to say: The food was awesome!