Importance & Advantages of cash book are as under
(i) Saves time and labour: When cash transactions are recorded in the journal a lot of time and labour will be involved. To avoid this all cash transactions are straight away recorded in the cash book which is in the form of a ledger. 
(ii) To know cash and bank balance: It helps the proprietor to know the cash and bank balance at any point of time.
(iii) Mistakes and frauds can be prevented: Regular balancing of cash book reveals the balance of cash in hand. In cash the cash book is maintained by business concern, it can avoid frauds. Discrepancies if any, can be identified and rectified.

Cash column of cash book will always have a debit balance because actual cash payments cannot be more than the actual cash in hand.

(i) A three column cash book includes three amount columns on both sides, i.e., cash, discount and bank, This cash book is prepared in the same way as simple and double column cash books are prepared.
(ii) Opening balance, cash deposited into bank, cheque deposited in the bank appear in the bank column on the debit side of the three column cash book.
(iii) All the transactions which decrease bank balance are recorded in the bank column on the credit side

Posting of entries in the petty cash account:
(i) When petty cash is advanced at the beginning:
A separate petty cash account is opened in the ledger. When advance is received by the petty cashier, petty cash account will be debited and cash account will be credited.
(ii) When individual expenses column are periodically totalled. The total of various petty expenses are debited and the petty cash account is credited with the total of the payments made.
The petty cash account will show the balance of cash. This balance will be shown in the balance sheet as part of cash balance.

Cash book: It is a special journal (subsidiary book) which is used to record all cash receipts and cash payments. It is a book of original entry or prime entry since transactions are recorded for the first time from the source document.
The features of cash book are:
(i) It is one of the subsidiary books.
(ii) It is a book of original entry.
(iii) The cash book is a ledger. It is designed in the form of a cash account.
(iv) It records all cash receipts on the debit side and all cash payments on the credit side.
(v) It always shows debit balance, as payments can never exceed cash available.

(i) Simple petty cash book:
(a) A simple petty cash book resembles the single column cash book. But the ruling is different.
(b) On the debit side, only one amount will be recorded which is the advance received from the head cashier.
(c) On the credit side, all payments are recorded in only one column. This is known as simple petty cash book.
(ii) Analytical petty cash book:
(a) In analytical petty cash book, a separate column is provided for different heads of payments and one column for total payments.
(b) When the petty expenses are recorded in the total payment column, the same amount is also recorded in the appropriate expense column. This is known as Analytical petty cash book.