Is it correct that individual accounts of customers is debited by the respective amount and the total of the sales book is posted to the credit side of the sales account in the general ledger ?
Yes, sale is revenue account and therefore, following the rule ; increase in revenue is credited' sales account is credited. Since, the sales book contains records of credit sales, the purchaser's account (being debtors) is debited following the rule applicable to asset accounts i.e., 'increase in assets is debited'.