Public Book (of orders)

The public book is the collection of buy and sell orders from ordinary market participants that are sitting away from the current price, waiting to be filled. Think of it as a queue: someone wants to buy a stock at $49.50 when it's currently trading at $50, or sell at $50.75 — those orders don't execute immediately, so they get parked in the book until the market moves to them or someone matches them directly.

Historically, on exchanges that used a specialist or designated market maker system, this book was managed by a single official (a specialist or "order book official") who held the orders and had visibility into exactly what was resting at each price and in what size. Other participants often could not see the full depth — only the best available price to buy and the best available price to sell, known as the bid and the offer.

The nuance that trips people up is that "the book" today usually does not mean one person's private ledger anymore. Most modern markets are fully electronic, and what traders call the book is an order book or depth-of-market display showing multiple price levels of resting buy and sell orders, often visible to everyone with the right data feed. The old model — a specialist quietly holding orders nobody else could see — largely describes a bygone market structure (notably old NYSE/Amex-style specialists or certain options market makers), and mechanics vary a lot by exchange and by era, so this term can mean different things depending on when and where it's used.

Regardless of the exact mechanism, the underlying idea persists: there is always a pool of unfilled public orders sitting at prices away from the last trade, and they represent real supply and demand waiting to be triggered.