L2 ORDER-BOOK DEPTH — the live bid/ask ladder for any crypto pair, normalized across Coinbase, Binance.US and Kraken (auto-fallback, or pick a source). Returns bids/asks to your depth plus the analytics that matter: mid, spread (absolute + bps), book liquidity per side, and slippage estimates for $1k/$10k/$100k market orders both directions. Deterministic, no keys, ~1.5s cache. ?pair=BTC-USD&depth=50&source=auto