NEW YORK (CNN/Money) - Online retailer Buy.com said Tuesday it will offer all of its books for at least 10 percent less than Amazon.com prices, in a move that continues an ongoing price war between the two companies.

Buy.com's offer takes effect immediately.

"They have 25 million customers, we have 5 million customers. We want to go directly after their customer base," said Robert Price, president and chief financial officer of Buy.com.

As examples, Buy.com noted that the fourth book of J.K. Rowling's Harry Potter series, "Harry Potter and the Goblet of Fire," costs $16.34 on Buy.com and $18.17 on Amazon.com. Some books are even cheaper when sold under Buy.com's 50 percent off sale -- with such titles as "Sum of All Fears" by Tom Clancy priced at $3.99 on Buy.com and $7.99 on Amazon.

Buy.com announced a package offering free shipping and no minimum purchase earlier in June, one day after Amazon lowered its free shipping minimum to apply to all purchases greater than $49.

"We took our free shipping truck and ran over their obstacle," Price said.

Price said Buy.com is considering several other plans to woo customers away from Amazon, but declined to reveal details.

Amazon.com (AMZN: down $1.01 to $16.50, Research, Estimates) could not be reached for immediate comment.