Toshokan Sensou is one of my all-time favourite shoujo manga, [[apart from Akagami no Shirayukihime, Oresama Teacher, Tokyo Crazy Paradise, Skip Beat!, Saiunkoku Monogatari (which is not so shoujo-ish) and few other titles.]]
I don't know what to say, the romance between the main characters (AND side characters!!) is just so sweet!! I couldn't stand the suspense and went ahead to read the novels. That wasn't enough either, and I proceeded to check out the anime and movie.
They all follow the same plot, but to be honest my favourite would probably be the manga, followed very very closely by the light novel.
The war over books may feel a bit silly as a prospective setting, but when you read I personally think it's easy to ignore all that.
Also, the art is absolutely amazing. I used to really like (well, still do) Tanemura Arina-sensei's and Haruta Nana's art styles, but this is just... I just LOVE IT!! It's perfect for shoujo manga in a way that isn't all overly sparkly like Tanemura-sensei's.
The romance may be a little slow-paced, but every single "moment" is so... sweet (?) it always has me rolling about screaming. Aah, I've never felt such accomplishment for the characters after following through with their romantic developments!!
I also like how it's a romance that somehow isn't focused on romance. Most of the time, it's hard to really balance out the romance when action is the main point of a storyline, but in Toshosen, even though what all the characters really think about are books and libraries, romance somehow just keep popping up everywhere in such perfect moments I don't even know how to describe this amazingness.
I may be exaggerating, but nonetheless, I don't care, because I LOVE THIS SERIES to bits!!