A crypto order book is a live list of orders waiting to be filled. Bids are buy orders sitting below the current price, asks are sell orders sitting above it, and the gap between the best bid and the best ask is the spread. Each row shows a price and the size waiting at that price, and many exchanges also show a running total, which adds up size from the best price outward. That running total is the quickest way to judge depth: it tells you how far through the book a given order size would reach before it ran out of resting orders at nearby prices.

Start every read at the spread. If the best bid is 100.00 and the best ask is 100.10, the spread is 0.10, and a market buy of 30 units that finds only 15 units at 100.10 and 15 at 100.20 will fill at an average of 100.15, not 100.10. That difference is slippage, and it grows when the book is thin. A deep market has large size stacked close to the current price, so a sizeable order fills near the displayed price. A thin market has little size nearby, so the same order pushes through several levels. Before placing a larger order, compare your size with the size within one or two percent of the current price: small relative to that depth means a clean fill, large relative to it means you should expect to move the price.

Bid and ask walls need special care. A wall is an order far larger than surrounding levels, for example 50 BTC bid at one price while nearby levels show one to five BTC. It can act as temporary support or resistance, because a large amount must be bought or sold to pass it. But walls can also be fake. Large orders are sometimes placed to influence sentiment and cancelled before execution, a practice known as spoofing, which is illegal in regulated markets but still occurs in crypto. A wall that appears and vanishes when price approaches it was never real supply or demand. Judge the whole shape of the book, not one row, and watch whether size replenishes after it is eaten or disappears on approach.

Remember what the book does not show. Each exchange has its own order book, so liquidity is fragmented: depth on one venue says nothing about depth on another. Hidden orders, iceberg orders that only display part of their size, and stop orders waiting for a trigger price are absent from the displayed book. A level also groups displayed quantity at one price, but units may be quoted in coins, contracts, or quote currency depending on the venue, so check the unit before comparing sizes. Practice on highly liquid pairs such as Bitcoin and Ethereum first, where books are deep and price action is clearer, before trusting order-book reads on thin altcoin pairs.

Use the book as an execution tool, not a fortune teller. It answers practical questions: what will my order probably cost, where is nearby support or resistance, and is this pair liquid enough for my size. It does not predict direction, and a bid-heavy book can still fall while an ask-heavy book can still rise. Combine depth with your plan: use limit orders when you refuse to pay through a thin book, accept partial fills or no fill as the price of that discipline, and never size a trade on the assumption that a visible wall will hold. The book shows intentions at this second. Intentions change.

A practical exercise builds this skill faster than reading about it. Open a liquid pair, hide your profit and loss display, and spend ten minutes each day writing down the spread, the size within one percent of price, and the largest wall on each side, then note what price did over the next hour. After two weeks you will have a personal record of how often walls held, vanished, or were eaten through, and that record will teach you more about your exchange's book than any generic rule about walls ever could.