I recently did an exercise on analyzing the limit order book for predictors of the first event that will occur in the next 1 second. There are 3 events: bid level retreating, ask level retreating, and no change in the top of the book price.
The result was that the log of (Bid/Ask) gave the strongest prediction.
Below is a scatter plot for a small sample that shows the relationship between the bid and ask quantities, and the corresponding colours indicate the outcome (grey = no change, blue = bid retreated, red = ask retreated). Contours indicate the probability estimated by multinomial logistic regression.
Here's the full version using tick data of the entire day (about 127,000 data points). White = no change, for better visibility and truncated at size=300.
Below is the full version, without the ax truncation.