Online retail sales rose a solid 9 percent in the third quarter, a healthy sign for Web merchants as they enter the holiday season.

Online retail spending rose 9 percent to $32.1 billion for the three months ended in September, compared to the same period a year ago, according to a comScore Inc. report released Monday.  It is the fourth consecutive quarter of positive year-over-year growth, after a year of flat or declining sales, said the Chicago-based digital research firm.

The top performing categories included books and magazines, computers and computer software and consumer electronics.

ComScore Chairman Gian Fulgoni called the third-quarter performance a “fairly positive indicator” for the holidays. But he also warned, “until the economy begins adding jobs at a meaningful rate, the lack of spending power among consumers will continue to be a drag on purchasing.”

Online retail sales rose 9 percent in the second quarter and 10 percent in the first quarter, helped by comparisons with flat to declining sales in the first three quarters of 2009.

The biggest 25 online retailers accounted for 70 percent of the dollars spent online, up 5.5 percentage points from a year ago. Merchants that sell their goods only online represented 58 percent of the industry revenue while retailers that sell both in stores and online made up the remaining 42 percent.