Your patrons may love them. Your library may host them. You may even have participated in a few yourself. Fandom conventions and fandom spaces are becoming increasingly popular in the cultural landscape, and fiction centered around them is flourishing!
For the uninitiated, "fandom" as a concept is a subculture around expressing your love for a particular media franchise.  Fans congregate for conventions to show off costumes they've made of their favorite characters, attend panels hosted by the creators of their favorite shows or games, or buy merchandise. If you've ever read any of those listicles showing off the best cosplays from San Diego Comic-Con, that's fandom at work!
Fandom can also exist in an online space, where people from around the world can connect in internet communities to talk about their favorite characters, relationships, or stories. They may even write fanfiction in the universe of the franchise. 
At the end of the day, books about fandom are all about connection: the characters use their media interests to discover new things about themselves, bolster their friendships, and maybe even fall in love.
Some popular titles that fall under this fandom theme include Ship It by Britta Lundin, in which a fan blogger finds herself at the center of controversy surrounding LGBTQ representation on her favorite show when she's brought on as a consultant; and Queens of Geek by Jen Wilde, where three Australian teens go to a fan convention in California and embark on a journey of self-discovery.
We've added this theme for books at the older kids, teen, and adult level.

Jamie Langer is a Metadata Librarian I at NoveList. She is currently reading This Is How You Lose the Time War by Amal El-Mohtar and Max Gladstone.