In a legal context, book of business usually refers to a lawyer’s or law firm’s client base. These clients can be categorized according to a specific legal field or the lawyer’s area of specialization.
Books of business often contain clients’ names, contact information, details of their contracts/cases, etc.
Books of business are especially important in determining and avoiding any conflicts of interest.
They also come into play when individual lawyers in a law firm each have their own book of business, which they may take with them if they leave the firm. This is an indicator of whether their business is portable or not.