Yes I've. It's not very long though. If you like Dan Brown books you'll find this interesting. Though I should warn you that the plot is quite hastened at the end and kind of leaves an open ending ( according to me; trying real hard to not give spoilers during explanation here!)
I haven't read the books so can't say anything about them. I'd give it 6/10.
I think Kora is for a more mature audience and the poor airing didn't do it justice. Avatar : The last air bender is a classic. But Ang had it easy compared to Korra. Where's Ang had clear motives and a known prick of a villain and good friends Korra didn't have such luck. It's really good to see Korra dabble in the greys trying to make a middle ground attainable. Not to mention the fact that it for the first time showed a LGBTQ representation in mainstream popular kids show makes Korra a good show on it's own. In my personal opinion Korra is kind of for like those who grew up with Aang.