Some groups focus on certain types of content. There are a few for travel, for example. There's also a pretty major one #SteemSTEM for STEM content, AKA Science, Technology, Engineering, Mathematics. Because of them, STEM content is one of the only high effort types of content that is often well rewarded on here. I'm not aware of one for book reviews though. There might be a curation group or community for reviews, but I don't know of it.

I've never done a photography challenge. I have done some writing challenges. I might have picked up a few followers from some of them, but not too overly interactive ones.

Communities are probably the best place to meet people. Most of the communities right now are on different discords and many aren't that talkative. I've picked up most of my most interactive followers either on some random post, or on steem.chat.

Steem might seem massive, but it's still really small compared to many sites. That's why it doesn't have a big book community yet. Many of its users aren't super active anymore either. Basically, we all gotta build this place up together.