Lone Wolf by Robert Muchamore
Fay has spent eighteen months locked up in a Secure Training Centre. Drug deals and rip-offs are the only things this teenager knows. Now she's back on the street, looking to settle old scores. CHERUB agents Ryan and Ning need Fay's knowledge to unearth a major drug importer. They're trained professionals with one essential advantage: even experienced criminals never suspect that children are spying on them. But Fay's made a lot of enemies and she's running out of time ...For official purposes, these children do not exist.