What I've found in wallet_api to get order book:
get_call_orders
get_limit_orders
It's really difficult for api caller to calculate an order book from the raw information and it will be affected by market rule changes a lot. And we need to implement the same logic in all the program languages we want to use. Can we provide an api to get order book directly? Something like {asks:[{price:{}, amount:xxxx, optional order:{}}], bids:[{price:{}, amount:xxxx, optional order:{}}].