You shouldn't be surprised that Naruto, One Piece, and Bleach have light novel spinoffs. They're not prose adaptations of the manga or anime stories, but brand-new, original stories. In Japan, popular manga series become full-on multimedia franchise factories, and apart from anime, toys, and merchandise, prose spinoffs are also commonplace. It makes sense. Fans can't get enough of their favourite series and characters, so light novels – the Japanese term for Young Adult novels – offer side stories, spin-off stories featuring fan-favourite characters – are a logical spinoff.
Naruto: Naruto's Story – Family Day
Written by Mirei Miyamoto with art by creator Masashi Kishimoto, this light novel is the second in a series that deals with Naruto and his cohorts, who are now adults and parents. Four stories and an epilogue serve as an overview of how the characters have raised their kids while still doing all the Ninja Village stuff they always did. It's a neverending battle as life goes on, after all. This ties into the Boruto sequel series that's now running as a manga and anime. It shows how Naruto's job as the hokage has made him a rather crappy dad how neglects his increasingly wayward son. Naruto's friends don't always fare as well with their issues and their kids either, and the stories try to keep it light by combining comedic situations with ninja action. The main message is that parents need to be in touch with their emotions to avoid screwing up their kids.
One Piece: Ace's Story Vol. 2
Written by Tatsuya Hamazaki based on Eiichiro Oda's megahit series, this light novel series tells the origin story of Portgaz D. Ace, the adopted brother of series hero Monkey D. Luffy as he sets out to get fame and glory by taking down a rival pirate captain. This is pretty much fanfic for the hardcore where various characters from the series pop up to interact with each other in ways never seen in the manga or anime. Once again, the power of friendship and sheer stubborn tenacity win the day. If you like your One Piece, you'll find plenty to like in this book.
Bleach: Can't Fear Your Own World Vol. 1
Written by bestselling Light Novelist Ryohgo Narita playing in series creator Tite Kubo's universe, this is the first actual sequel to the manga series. The main character is not Ichigo Kurosagi but the supporting character Shuhei Hisagi, investigating a series of murders to uncover a new enemy who wants to take over the Soul Society. It's a fanfic fest again as Hisagi crosses paths with virtually every major character in the series. That's what the fans are here for, after all. It's all about sticking by your friends and the memories you made together.
These light novels are a perfect gateway drug to entice kids to read prose novels about their favourite manga and anime characters. As Shonen stories, they all have common themes, probably honed under the supervision of editors, namely that of the power of friendship and that everyone should really be nice to each other. It must be thanks to the editors and the supervision of the original creators that the light novels feel as close to the manga and anime series as possible. And anything that gets kids to read more is a good thing.