Coral Blocks has a free service that provides aggregated liquidity from Coinbase, Coinsetter, Okcoin and LakeBTC. You can get the aggregated book in JSON through the link:
http://www.coralblocks.com:45453/book/BTC-USD
{
"last updated": "20151015-21:01:27.551",
"time zone": "America/New_York",
"symbol": "BTC-USD",
"exchange": null,
"spread": -0.90,
"best bid size": 0.07000000,
"best bid price": 255.87,
"best ask size": 0.00090000,
"best ask price": 254.97,
"bids": [
[ "OKCN", 0.07000000, 255.87],
[ "OKCN", 0.07000000, 255.84],
[ "OKCN", 0.07000000, 255.81],
...
],
"asks": [
[ "LAKE", 0.00090000, 254.97],
[ "LAKE", 0.06700000, 254.98],
[ "LAKE", 0.60000000, 254.99],
...
]
}
You can also get real-time updates through TCP and FIX. More info about the API here:
http://www.coralblocks.com/index.php/2015/10/free-real-time-bitcoin-market-data/
Disclaimer: I am a developer at Coral Blocks.