Private ledger is basically such book of accounts in which such accounts are maintained which are maintained to be confidential. Therefore, any such account which might cause some privacy issues or the one that is crucial for the owners’ of the business than such account is maintained under Private ledger.
It cannot be treated as a new type of ledger in account however, we can surely say it as a new category of ledgers. Because it does not contain some new kind of accounts in itself. It is just making few accounts private or shifting them to such ledger to which access is restricted and only owners’ can access.
Private ledgers might be present since the present shape of accounting has been developed but these ledgers have become a notable addition after information technology became the part of accounting systems. As the bookkeeping, posting and approval functions are divided among different teams of accounting department thus owners might be interested in having few ledgers private and thus accessible only by them via digital interface.
Lastly, Private ledger does not mean ledgers of private companies 
