Amazon has fired the most recent shot in the pricing war with Wal-Mart that broke out Thursday, by introducing same-day delivery in seven major cities and expanding its Saturday delivery options. After cut the price of its top 10 pre-ordered titles to $10 (which includes free shipping) on Thursday, Amazon followed suit, and yesterday, both retailers dropped pricing on those 10 titles down to $9. Now, Amazon has announced it will begin offering same-day delivery in Baltimore, Boston, Las Vegas, Philadelphia, Seattle and Washington, D.C. (it already offers the service in New York).

Amazon’s same-day shipping charge is $14.99, plus $3.99 per book. The cut-off time for ordering with same-day delivery ranges from 10:00 a.m. local time in New York and Philadelphia, to 1:00 p.m. local time in Seattle. Girish Lakshman, v-p of transportation at Amazon, said “We want to make online shopping as convenient as possible.” In addition to the same-day delivery service, the retailer will now deliver items ordered before the cut-off time on Thursday using two-day shipping on Saturday (instead of Monday).

Among the $9 titles is Going Rogue by Sarah Palin, due out from HarperCollins November 17. Amazon and Wal-Mart offer the book for $9, while and are selling it for $17.39. The book's list price is $28.99. Other $9 hardcovers include Kindred in Death by J.D. Robb (Putnam), The Lacuna by Barbara Kingsolver (Harper) and Ford County by John Grisham (Doubleday).