Girl, 15, Charming But Insane by Sue Limb (2004)

Absolutely adorable young adult novel about a young British girl living with her mum and struggling with all the usual teen troubles--boys, friends, etc, but the dialogue (mostly in the voice of the heroine Jess Jordan) is beyond witty. Plus, Mom's a librarian! See awesome quote.
“Unfortunately she met her mum by the gate, and she could tell by her face that Mum had had one of those days. Occasionally people came into the library and peed, pooed, or got drunk and starting shouting abuse. Drunks and vagrants went to sleep in the reference section. Once a very old man who lived on the streets had died on the Oxford English Dictionary. You might think that being a librarian would be a quiet, cushy job, but sometimes it seemed that the library was really a nightmarish extension of the mean streets and that librarians were just cops and paramedics disguised in tweedy cardigans and long dangly parrot earrings from the charity shop.”