What is an order book?
The order book is the live list of every resting buy order (bids) and sell order (asks) in a market, arranged by price. It is the market's actual supply and demand at this second — the thing the chart is only a shadow of.
How to read it
Bids stack below the current price, asks above. The gap between the highest bid and lowest ask is the spread; the size resting at each level is depth. A market order consumes the book level by level — which is why large orders in thin books fill at worsening prices (slippage). A limit order instead joins the book and waits to be hit.
What depth tells a trader
A thick book absorbs big orders with little price movement; a thin book turns modest orders into large candles. Depth also changes by the hour and by mood — it evaporates during panics, exactly when cascading liquidations fire waves of market orders into it. Forced sellers meeting a vanishing book: that combination is the mechanism behind crypto's trademark wicks.
Honest caveats
The visible book is not the whole truth. Orders can be spoofed — placed to be seen and cancelled before filling — and much real liquidity hides in unshown iceberg orders and market-maker algorithms reacting in milliseconds. Beginners should read the book for spread and rough depth, not as a crystal ball.
FAQ
Do I need to read the order book to trade? For higher-timeframe trading, no — checking the spread before entering is enough. Book-reading matters mostly for short timeframes and large size.
Why did my market order fill at a worse price? It ate through the first level of the book and kept going — that's slippage, proportional to your size versus the depth.
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