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.

In the past, Apple (AAPL) has said that many more songs will be priced at 69 cents than at $1.29. But that 69-cent bargain bin is viewed as little consolation to some who question the wisdom of introducing what amounts to a 30 percent price increase with the economy in recession and online piracy an easy option.

Said Nine Inch Nails manager Jim Guerinot, “Wouldn’t it make sense to try to price it cheaper instead of squeezing the handful of people who are still willing to pay for music?”

Perhaps. But when has the recording industry ever done anything that made sense?