"blockchain_market_order_book <quote> <base>" shows the order book. The order of the currencies given is important - if you receive an "invalid market" message, try swapping the currency symbols. Example:
>>> blockchain_market_order_book USD BTS
BIDS (* Short, + Relative, - Relative Limit) | ASKS
TOTAL QUANTITY PRICE | PRICE QUANTITY TOTAL COLLATERAL
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
849.9251 USD 100,291.17310 BTS 0.008474576271 USD*| 0.008983918785 USD 3.07216 BTS 0.0276 USD
463.3051 USD 54,670.00194 BTS 0.008474576271 USD*| 0.008984725966 USD 2.30392 BTS 0.0207 USD
...
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SHORT WALL | MARGIN
TOTAL QUANTITY INTEREST RATE (APR) | CALL PRICE QUANTITY TOTAL COLLATERAL EXPIRES
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0.004026912700 USD 18,624.67120 BTS 99.9999 USD 5 days in the future 37,249.34241 BTS
...
Maximum Short Price: 0.00905770052507467 USD / BTS
Note that the base and quote currencies are swapped as compared to the GUI. This also means the prices are inverted, and you have to think of buying/selling BTS for BitUSD.
In the above example, on the ASK side someone is trying to sell 3.07216 BTS at a price of 0.008983918785 USD/BTS for a total of 0.0276 USD (so actually he's trying to buy .0276 USD for 111.31 BTS/USD). On the BID side, someone is trying to buy 100,291 BTS at 0.008474576271 USD/BTS (so actually he's trying to sell 849.9251 USD at 118 BTS/USD). The asterisk on the BID side indicates that this is not a normal BID but a SHORT.
At the bottom, on the left side you can see the SHORT WALL (i. e. all shorts currently executing at the price feed), and on the right side an incomplete list of open cover orders with their margin price and expiration time. The last line contains the price feed.
So, if you want to buy some BitUSD from the short above you have to place an ASK order like this:
wallet_market_submit_ask <account_name> 291 BTS 0.008474576 USD
Similarly, to sell your BitUSD to the above ask you have to place a BID:
wallet_market_submit_bid <account_name> 3.07216 BTS 0.00898392 USD
Note that due to rounding issues it is a good idea to bid a slightly higher price and to ask a slightly lower price.