Vandana, Google considers downloading, streaming free content as a transaction as well, even if its selling price is zero. You either need a Google Wallet account or need to enter your card details in order to download a free e-book.
Your card details should be safe at Google and there won't be anything charged to your account. A friend of mine not too long ago was charged a small amount when he registered his credit card on Google, however that amount was immediately refunded back to his account.
I maybe wrong, but a debit card is also accepted on Google now.