If you wish to restart your organization, you can either delete all the transactions in each module or create a new organization and key in the required data there. To delete transactions, you can move on to each module (such as Invoice, Sales Order, Bills, Expense, etc.) individually and bulk delete the transactions there.
Note: You will have to delete the transactions bottom up i.e, if credits/payments have been applied to invoices, you would have to delete the credit/payment first, delete the invoice, and then delete the customer eventually.
Books
Online accounting software
for small businesses.