An Introduction to Limit Order Books

Is market order slippage risk still practical? In the case of stock trading liquidity appears to be abundant for most stocks.

Off-topic, but would love to know what tool the author used to build that site/page. Is it some sort of platform/tool, or something they wrote up themselves? I really liked the side bar table of contents, along with the general format.

What happens when a buy limit @10 order meets a sell limit @10.5 order?