Well that has me sold. Hopefully it will be released over here, I'll probably start getting the manga again as well.It's second tier canon to the games and interviews...some people used to split it off as third tier but the interviews have become something like required reading for the series now so they get a little bump up to second.
Anything revealed in them that isn't contradicted by the games is canon. It was in the novels that it was confirmed that when you kill the Heartless and Nobody, the Somebody will return. It was also the first place it was hinted that Axel had come back to life/became a Somebody again after his demise in KH2.
They gave a backstory to Vanitas, and some try to write it off as a Kanemaki's way of giving sympathy to him but it adds a lot of depth to Vanitas that was missing in the games. Also weird, fun fact: people that lean towards the light love sweets, when people who lean closer to the darkness prefer something more bitter or less sugary.
It also gives some great insight into Sora's mind during the game's events. He's very insightful at times, sometimes the things that sounded like Sora was being obtuse really weren't at all, so it is nice to see what makes him tick.