Bungo Stray Dogs is both. It began as a light novel and then was transformed into a manga. The light novel provides detailed storytelling, while the manga brings the characters and scenes to life visually.
Well, in the 'bungo stray dogs' series, each volume usually has a central conflict. Maybe in volume 15, it could be related to the power struggles between different groups of ability - users. It might also focus on the development of certain characters, like how they face new challenges and grow. However, without reading it, this is just speculation.
Typically, characters like Dazai Osamu are central to the 'bungo stray dogs' series. In volume 15, he might still play a major role. Also, characters who are part of the Armed Detective Agency and the Port Mafia are likely to be prominent. For example, Kunikida Doppo from the agency, who has his own set of ideals and abilities. But again, this is based on the general pattern of the series and not definite for volume 15 without reading it.
One of the main themes is the exploration of special abilities. Each character has a unique ability which often reflects their personality and backstory. Another theme is the struggle between good and evil, as the characters are often divided into different factions with different moral stances. Also, friendship and loyalty play a big role as the characters form bonds in the face of various challenges.
The number of Bungo Stray Dogs light novels is not fixed. It keeps increasing as new ones are published. You can check the latest information on official bookstores or the publisher's website.