Books Of Accounts

It depends on the nature of business and practice of maintaining records
if your sales and purchase are 100 percent online.. then physical documents are not necessary..excepts payments in cash for that you need to maintain cash vouchers and if you are issuing and receiving manual invoices than you need to maintain following basic records

sales invoice in sequence
purchase invoices date wise
cash payment vouchers
cash receipt vouchers
bank book if maintained in software
ledger confirmations of debtors and creditors
fixed assets purchase records
copies of contracts and deeds etc
which are necessary to prove the genuiness of transaction as per the nature of business

As per section 129
u can maintain all books of accounts in electronic mode but keep it in india or they are orignal and for cash transaction u should maintain cash payment or recipt voucher