The Irregular. The Hollow Universe

London, 1894. Fog rolls thick down Baker Street, gaslights flicker, and somewhere in the shadows, a detective everyone thought was dead has just come back. But this isn't Sherlock Holmes's story - it's Fen Sparrow's. Fen is twelve, quick-fingered, sharp-eyed, and the newest member of the Baker Street Irregulars - the ragtag gang of street kids Holmes hires when even Scotland Yard can't go where children can.
When a string of strange crimes, coded messages, and disguised villains starts crisscrossing the city, Fen is pulled into a mystery bigger than any errand: a secret society, a stolen cipher, and a traitor hiding in plain sight. Packed with detective work, hidden clues, Victorian London atmosphere, narrow escapes through foggy alleyways, disguises, secret codes, loyal friendship, and the thrill of outsmarting adults twice your age, this is a fast-paced adventure for young readers who love puzzles, courage, and a hero the grown-ups keep underestimating.