Book Publishers Beware! At iTunes, Expensive Music Equals Slower Sales.

After years of complaints, last year the music labels finally got what they wanted from Apple–the ability to raise prices on their songs. Last April, iTunes introduced a “variable pricing” scheme, which gave the labels the ability to move prices from 99 cents a song to $1.29 (and for some tracks, down to 69 cents).

The result? Music sales are slowing.

Warner Music Group (WMG) said this morning that it has seen unit sales growth at Apple’s (AAPL) iTunes decelerate since the price increase: Industrywide, year-over-year “digital track equivalent album unit growth” was at five percent in the December quarter, down sequentially from 10 percent in the September quarter and 11 percent in the June quarter.

And since iTunes sales make up the majority of Warner’s digital revenue, growth is contracting there, too. In the last quarter, digital revenue at the label was up eight percent compared with a year earlier, when that number was 20 percent.

The positive spin here is that music downloads are a “mature” business anyway. So by raising prices, the labels are simply extracting whatever value they can.

And indeed, Warner CEO Edgar Bronfman Jr. argued that the pricing change has been a “net positive” for Warner. But he also suggested that in hindsight, perhaps it wasn’t a great idea to raise prices 30 percent during a recession.

So here’s the question for the book industry, which has been working very hard to boost the price for its digital goods: Which lesson do you learn from this?

My gut is that the industry will see this parable the way Bronfman apparently does: If you can move prices up early in the digital adoption cycle, you’re much better off.

During the earnings call, Bronfman sounded a bit wistful as he noted the book industry’s apparent success, with the help of Apple, at raising prices above the $9.99 floor Amazon (AMZN) had set. “It’s interesting that the book publishing industry, on the iPad, has much more flexibility than the music industry had,” he noted.

The counter here is the one that seems obvious to everyone else: Lower prices and you can sell more stuff. Looks like we’ll be getting another real-world test of this economics lesson soon.