Perhaps The Last Book in the Universe by Rodman Philbrick?
They are not injecting books in the novel, per se, but there are mind-probes, I think they were spinal taps, used for directly downloading entertainments and such, more analogous to movies or video-games. The main character is a young boy who can't use them, because of a preexisting medical condition (epilepsy) - his "special power", perhaps, because I recall the mind probes have many dangerous and negative side effects.
The book starts with the main character, Spaz, and his adoptive family. He is also sent on a mission to find an old man who was hiding books (the "lost arts" or secrets of literacy and literature) - because they were valuable, because that knowledge was lost, because, I think, reading did something actually opposed to just using mind-probes, maybe encouraged thinking instead of just receiving entertainment?
The end of the novel has Spaz and a few other characters, one of whom was a "special" girl who belonged there, going to the hidden or forbidden city of "proovs", improved people, who had a beautiful society. I don't recall if it was actually underground, or just walled off, though. Those who came in with the group who weren't already improved got kicked out, and after that was the general revelation of why the mindprobes were bad (brain damage or something), so the mind probes got shut off (to the unhappiness of those using them), general themes of society having to restart, and the book ends with Spaz finishing the old man's work of writing a book, the title "last book" since no one else learned the secrets.
It isn't a perfect match, but it seemed like a fairly close fit. I hope this helps.