Introduction:
The ledger book is an essential tool used in accounting to record financial transactions. It serves as the principal book of accounts where all the transactions from various subsidiary books are summarized. In this response, we will explain why the ledger book is popularly known as the principal book of accounts.

Explanation:
The ledger book, also known as the general ledger, is a book that contains all the accounts of a business entity. It is a complete and organized record of all financial transactions, classified into different accounts. Here's why the ledger book is considered the principal book of accounts:

1. Centralized Record:
The ledger book serves as the centralized record of all financial transactions of a business. It consolidates information from various subsidiary books and provides a comprehensive view of the company's financial position. It acts as a backbone for all accounting processes.

2. Summarizes Transactions:
All transactions recorded in subsidiary books such as cash book, purchase book, sales book, etc., are summarized in the ledger book. Instead of having scattered information in different books, the ledger book presents the financial data in an organized manner, making it easier to analyze and interpret.

3. Classification of Accounts:
The ledger book classifies accounts into different categories, such as assets, liabilities, equity, revenue, and expenses. Each account has a separate page in the ledger book, where all related transactions are recorded. This classification helps in understanding the financial position and performance of the business.

4. Posting from Subsidiary Books:
The ledger book is where the process of posting occurs. Posting involves transferring the summarized transactions from the subsidiary books to the respective accounts in the ledger book. This process ensures that all transactions are properly recorded and organized for future reference.

5. Trial Balance Preparation:
The ledger book is crucial for preparing the trial balance, which is a statement that lists the balances of all accounts at a specific date. The trial balance helps in detecting any errors or discrepancies in the accounts and ensures that the accounting records are accurate and balanced.

Conclusion:
In conclusion, the ledger book is popularly known as the principal book of accounts because it acts as a centralized record, summarizes transactions, classifies accounts, facilitates posting from subsidiary books, and is instrumental in preparing the trial balance. It is an essential tool for maintaining accurate and organized accounting records, providing valuable information for decision-making and financial analysis.