IS CASH BOOK SUBSIDRY OR PRINCIPAL BOOK?

Cash Book is both a Subsidiary book and a Principal Book.

Subsidiary Book- As all cash transactions are recorded directly in the Cash Book, so there is no need to record them separately in Journal. Also, with the help of these recorded transactions ledgers can be prepared, that is why, Cash Book serves the purpose of Subsidiary Book. 

Principal Book- There is no need to prepare Cash Account  separately as it itself act as cash or bank account and the closing balance is directly shown in the Trial Balance. That is why, Cash Book serves the purpose of Principal Book.