I haven't read the myself, but from the what I've read about the situation, the book is about the Shenzou and Enterprise crews working together, and we see Pike and Spock interacting with Burnham and Saru, but then when they show up in Discovery Season 2, Pike doesn't know them, and Spock and Burnham haven't seen or spoken to each other since they were kids. I think there have been some other smaller little bits that also don't really line up, but I'm not sure of the details.
Apparently at least part of what happened is that the book was written while Bryan Fuller was still in charge, and he was never going to have the Enterprise appear on the show, but he left and the new show runners obviously had different plans.