College textbooks are expensive. You probably already know this. A new biology or economics book can cost $300.

And prices have been soaring, doubling over the past decade, growing faster than the price of housing, cars, even health care.

But, surprisingly, the amount students actually spend on textbooks has not been rising. In fact, the best data we could find on this shows students have been spending a bit less over time.

How is this possible? Well, when prices go up, people usually try to find ways to avoid paying those higher prices. That seems to be what is going on here. The spread of the Internet has made it easier for students to find used textbooks in faraway places. Textbook rental has become a thing. Some students can now buy e-textbooks, which tend to be cheaper than print books. Others are borrowing books or going without.

That last chart actually helps explain the first one showing prices for new books going through the roof. If you're a textbook publisher selling fewer books every year, how do you cover your costs? One way is to raise the price for the new editions. Of course, this encourages students to buy even fewer. A former textbook salesman I talked to called it the "spiral of destruction."

One textbook executive told me the way out of all of this is to replace textbooks with something better and cheaper: educational software. Basically interactive, digital versions of textbooks.

For students there is one drawback, though. You can't sell digital textbooks back to the bookstore, or to anyone, at the end of the semester. There is no used market. That's another reason publishers like them.

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