As Ibuki and Toko are from different Danganronpa games, they never meet each other in the canon timeline.
They share some events at Danganronpa S: Ultimate Summer Camp. At Ibuki's Summer Festival events, she can call Toko by an affectionate nickname and invites her to lunch. But Toko resists, saying they aren't good enough friends for Ibuki to call her by a nickname. Ibuki responds by jokingly calling her Mistress Fukawa instead and insisting that they eat together. In Ibuki's final "My Future" event, she reveals that she had written a song for Toko, among several of their other classmates.
For similarities between the two, they can be considered alike. Both are girls who wear a sailor-type collar and top with a skirt. They also both have long hair. Both became famous at a younger age. One of Ibuki's songs became one of the highest-grossing singles in the country. Toko's novel So Lingers the Ocean became very popular and she won many awards. Before Hope's Peak Academy, both attended a girls-only High School. Both like the MonoMono presents Rose in Vitro, The Funplane, and Hope's Peak Ring.
Their character designs and aesthestics tend to be rivaling. Toko's is dark, and mainly brown. Ibuki's is very colorful, mainly being pink, blue, and black. This is a common trope. Interfering with this, Toko has a very long skirt. However, Ibuki has a primarily short skirt.
The two have extremely contrasting personalities that are what make the ship so attractive. Ibuki is a loud, obnoxious girl who exhibits joyful mannerisms and rarely wavers in her confidence. In comparison, Toko is very quiet, nervous, is very inexperience socially and has incredibly low self-esteem. The pairing creates a sweet romance or friendship. This is depending on how the fan views the pair.
The main reasons the pair is shipped is to create an alternate happy romance for both girls. Instead of Komaru helping Toko overcome her feelings for Byakuya, some wonder what would happen if that was Ibuki. Fans believe that Ibuki would be a great way to have Toko come out of her shell and create a more healthy and happy version of herself. On the other hand, Ibuki, who dislikes books and reading, may begin to like them with aid from Toko, who is enthusiastic about them.
Since Toko and Ibuki never interact, Music Book is a rarepair. The ship is sometimes seen as an alternate version of Tokomaru, where Komaru is swapped out for Ibuki. Being a very uncommon ship, it has many ships more popular than it, including the aforementioned Tokomaru, as well as Band Aid.
Another reason people ship this is because of Toko's love for Byakuya. Also because of Ibuki's ship with the Ultimate Imposter who is impersonating Byakuya. So some fans like to ship these two seeing them as rivals competing against which Byakuya is better.
- The ship name "Music Book" comes from Ibuki and Toko's talents of Musician and Writer.