The book Leia, Princess of Alderaan begins when Princess Leia Organa turns sixteen years old. It seems to take place over a matter of months, or a year even. Leia is around sixteen when we see her in the Star Wars Rebels episode "A Princess on Lothal." How far away are the events of the book from the events of that episode. There wasn't even a passing reference to it in the book.
The book seems to choose to remain separate from the show series. However from the events of the book we can figure out that it must certainly have occurred before the events of A Princess on Lothal. This is because in the book Leia only begins to learn of the Rebellion and receives hints of it, however in the episode she's is clearly an included member, and sent by her father to help Phoenix Squadron.
According to this timeline in the forum for the Star Wars wiki, it suggests that The Future of the Force and Legacy take place shortly before the events of A Princess on Lothal, we can presume that Leia, Princess of Alderaan takes place during the events of the preceding episodes, possibly up to A Day's Duty. This would mean that the novel ties in about half way into the second season of Star Wars rebels, during the events of The Future of the Force and Legacy.