I lived in Shanghai and Beijing for five years, in my experience, there really isn't much in the way of regulation of Dharma books, depending on how you look at it.
Dharma books which are in any way political could cause issues - e.g. Books referencing the Chinese occupation of Tibet, books showing a map of China with Tibet as separate country etc.
Anything else would be pretty much fair game. I've met Lama's who have had dozens of books published in the mainland.
It's worth noting that there tends to be a very large gap between what's regulated in China, and what's actually enforced. If for some reason a popular Dharma book was banned in China, people would just grab a Taiwanese or Hong Kong version off the gray market.