Sales, reported earlier, rose 10.5percentt.
Retail sales, reported last week, increased 0.3percentt after rising 1.5percentt in April.
The Land Registry, which logs actual sales, reported a 0.1% increase overall, with big regional differences.
Rankings reflect sales reported by vendors offering a wide range of general interest titles.
The figures were based on US print unit sales reported to Nielsen since the novel was published in July.
Houlihan/Lawrence's report covers sales reported by the Westchester Multiple Listing Service as well as those in their own offices.
Revenues were $7.4 billion to $7.5 billion, two-thirds of the $11.2 billion in sales reported during the third quarter of 2000.
They fell, but by less than 1percentt, or a decline of 11 sales out of nearly 3,500 sales reported in April 2007.
They noted that February and March retail sales were revised downward and that while automotive sales rose last month, unit sales reported by the industry fell.
Land Registry gathers all of its data for sales reported during a month, including sales completed in previous months, and calculates average prices.
In 1998, after the law took effect, the number of profitable sales reported fell to 111,000, but the average gain did not change, according to the I.R.S.
