In straight numbers, for non-fiction authors who are experts in their topic, it's good to see social media numbers that start at a minimum of 20,000 likes/followers on social media and a slightly higher number of unique visitors to a website/blog. For fully branded authors, non-fiction or fiction, it would be reasonable to see three times those numbers.
There are also layers to what's challenging. I would say working in such a competitive market: between self-pub and pressures to produce, produce, produce, the book market is flooded and we're all trying to find books that can rise above the white noise. And then there's communicating with authors about the business end of publishing-- they have a dream for their books and the publishing marketplace isn't necessarily conducive for making all dreams come true. A book that's not selling can be brutally disappointing for everyone, including publishers.
If you meant trends in topics, then I would say that LGBTQ is huge. Narrative non-fiction on natural sciences and social sciences. Inspirational stories without any other agenda but to inspire. No-BS books…on everything from leadership and personal growth to dating and tidying up. And vampire-zombie hybrids, of course.