async subscribeOrderBook (symbol, prec = 'P0', len = '25') {
    return this.managedSubscribe('book', symbol, { symbol, len, prec })
  }
Subscribe to an order book data channel

@param {string} symbol - symbol of order book
@param {string} prec - P0, P1, P2, or P3 (default P0)
@param {string} len - 25 or 100 (default 25)
@returns {boolean} subscribed
@see WSv2#managedSubscribe
@example
await ws.subscribeOrderBook('tBTCUSD', 'R0', '25')