December sales increased 8.00% year over year, down more than four percentage points from the 12.31% year-over-year growth in November, among small and midsized retailers indexed by Dydacomp, a provider of order management software. Books continued to dominate sales growth, rising 51.31% from a year earlier, compared with a rise of 49.20% in November, as nearly all other categories in the index rose by less than 3% or showed declines.

The Dydacomp SMB Index, prepared for Internet Retailer, is based on data compiled from more than 1.5 million orders per month at more than 1,500 e-commerce operators across more than 15 product categories—accounting for more than $200 million in combined monthly retail sales. The retailers are users of Dydacomp’s order management software.

Fred Lizza, CEO of Dydacomp, figures the slower growth in December stemmed from consumers buying more of their gifts earlier in the holiday season. “The biggest surprise in 2012 was the strong upsurge in the beginning of the holiday shopping season, starting before Black Friday, then the let-down as we approached the end of the holidays,” Lizza says.

He notes two possible causes for the earlier shopping. For one thing, he says, the trend of more consumers shopping online has changed the pace of holiday shopping, with many web customers shopping online early in the holiday season to ensure timely deliveries. Second, he says consumers may remember the difficulties retailers had in recent holiday seasons keeping popular products in stock, and so shopped earlier. “The consumer attitude coming into the holiday season included an expectation based on the last couple of years, that there might not be a lot of inventory,” Lizza says. “They learned that if you want something, don’t wait to buy it.”

Other analysts also have pointed to Thanksgiving falling on its early possible date in 2012—Nov. 22—which meant there were eight post-Thanksgiving shopping days in November. In 2013, by contrast, there will be only two shopping days in November after Thanksgiving Day, which falls on Nov. 28.

But while overall sales growth cooled down from November, the sharp sales growth in books and in the index category listed as “Other” led overall sales of $307.99 million, the first time the index has topped $300 million, Lizza says. Lizza attributes the strength of the books category to rising sales at e-retailers selling religious and other types of special interest reading materials. The Other category includes merchants that sell products in numerous categories, and Dydacomp is not able to break out category sales for those merchants, Lizza adds.

In addition, average order values in December rose in all categories except for advertising promotions, consumer electronics and home furnishings; the average order value across all categories rose 9.67% year over year. Total transactions across all categories, meanwhile, dropped by 1.53%. Books stood out in all metrics, with the category’s average order value rising 29.53% and its number of transactions rising 16.81%.

Following are the dollar volumes in gross merchandise sales (in millions) across all retail channels for each product category in the index for December 2012, with the percentage changes over December 2011, and the change for all of 2012 compared with 2011:

Advertising Specialties/Promotional Items, $0.76; -20.17%, -6.73%,

Auto Parts, $13.23; 2.44%, 8.78%

Books, $64.27; 51.31%, 28.96%

Clothes, $20.38; -14.25%, -4.64%

Consumer Electronics, $7.67; -16.97%, -1.96%

Educational Training Materials, $2.79; -17.33%, -2.33%

Food/Health, $75.07; -4.14%, 4.11%

Home Furnishings, $4.32; -17.43%, -4.73%

Jewelry, $1.06; -22.47%, -24.61%

Music/Video/DVD, $1.53; -14.06%, -8.09%

Sporting Goods, $8.62; -1.17%, 22.32%

Tobacco Products/Alcohol, $1.78; 6.31%, -0.15%

Tools/Hardware, $2.65; -9.99%, -7.41%

Toys, $36.74; 2.95%, 2.64%

Other, $67.12; 18.30%, 11.37%

Total, $307.99; 8.00%, 8.38%

Following are the average order values across all retail channels (including contact centers and a small number of physical stores that use Dydacomp’s software) for each product category in the index for December 2012, with the changes in average order size over December 2011 and the change for all of 2012 compared with 2011:

Advertising Specialties/Promotional Items, $70.97; -4.02%, 3.83%

Auto Parts, $234.86; 6.89%, 4.26%

Books, $125.27; 29.53%, 17.36%

Clothes, $94.94; 0.97%, 4.01%

Consumer Electronics, $208.12; -8.57%, 1.91%

Educational Training Materials, $133.64; 0.07%, -0.38%

Food/Health, $119.18; 5.78%, 3.52%

Home Furnishings, $108.02; -7.38%, -3.68%

Jewelry, $354.65; 48.07%, 9.55%

Music/Video/DVD, $113.14; 0.88%, 1.67%

Sporting Goods, $135.41; 9.35%, 13.21%

Tobacco Products/Alcohol, $108.31; 3.10%, -1.95%

Tools/Hardware, $207.78; 2.09%, 0.97%

Toys, $127.91; 6.65%, -2.51%

Other, $123.39; 11.28%, 3.89%

Total, $125.05; 9.67%, 4.79%

Following are the number of transactions across all retail channels for each product category in the index for December 2012, with the percentage changes over December 2011 and the the change for all of 2012 compared with 2011:

Advertising Specialties/Promotional Items, 10,689; -16.82%, -10.17%

Auto Parts, 56,328; -4.16%, 4.33%

Books, 513,017; 16.81%, 9.89%

Clothes, 214,650; -15.07%, -8.31%

Consumer Electronics, 36,836; -9.18%, -3.80%

Educational Training Materials, 20,867; -17.38%, -1.96%

Food/Health, 629,897; -9.38%, 0.57%

Home Furnishings, 40,019; -10.85%, -1.08%

Jewelry, 2,999; -47.64%, -31.18%

Music/Video/DVD, 13,549; -14.82%, -9.60%

Sporting Goods, 63,671; -9.62%, 8.05%

Tobacco Products/Alcohol, 16,441; 3.12%, 1.83%

Tools/Hardware, 12,741; -11.83%, -8.30%

Toys, 287,231; -3.47%, 5.28%

Other, 543,965; 6.31%, 7.20%

Total, 2,462,900; -1.53%, 3.43%

Dydacomp notes that performance numbers in its SMB Index tend to be lower than broader e-commerce market figures because the index does not include growth at the largest online retailers. Some large retailers, notably Amazon.com Inc., No. 1 in the Internet Retailer Top 500 Guide, are growing much faster than e-commerce as a whole. In addition, information in the index includes a small percentage of sales processed in stores and contact centers using Dydacomp’s order management software, and in many cases such sales are declining, the company says.

Favorite