The question requires determining which statement about accounting is correct. Let's evaluate each of the given options:

1. Agreement of trial balance is a conclusive proof of the accuracy.
   • This statement is incorrect. The trial balance is drawn to check the arithmetic accuracy of the ledger accounts, i.e., to ensure that credits equal debits. However, its agreement does not guarantee absolute accuracy as errors like omission, commission, or compensating errors may still exist.
2. Suspense account opened in a trial balance is a permanent account.
   • This statement is incorrect. A suspense account is temporary and is used to balance out discrepancies in the trial balance until the errors are found and resolved.
3. At the end of the accounting year, all the nominal accounts of the ledger book are balanced.
   • This statement is incorrect. Nominal accounts are not balanced but rather closed. At the end of the accounting period, the balances of nominal accounts are transferred to the profit and loss account.
4. A ledger is known as the principal book of accounts.
   • This statement is correct. The ledger is indeed known as the principal or main book as it contains all the accounts in which transactions recorded in general entries are posted, hence showing the financial position of the business.

Thus, the correct answer is: A ledger is known as the principal book of accounts.