The collection of all resting bids and offers across every visible price level, visualised as a price ladder with resting bid quantity on the left side of each price and resting offer quantity on the right. Only the area around the current market price is typically displayed (Level II data shows roughly five to ten levels deep on each side). Book depth is dynamic — orders are added, cancelled, and modified hundreds of times per second — so the LOB reveals current resting interest, not committed intent.