Essentially, front book prices are the prices of services available to new customers, and back book prices are all the prices that were previously available to customers, stretching back over time.
经过查证，知道back book和front book的意思，但却尚未找到对应的中文参考。不过既然有back和front互为反义词，那么，是可以直接翻译成"后端"、“前端”的，不过，由于是首次出现中文，可以加括号备注下back book的意思；或是不翻译字面意思，直接翻译为“老客户专享价”。
这里的book与书没有任何关系，应该是订货。
你根据前后文自己再琢磨琢磨吧。但意思肯定是这个意思。

back(adjective): of a past date; not current.

book: a volume in which financial or business transactions are recorded.