Self-made billionaire Bill Gates is an avid reader. He's known to go through about 50 books a year and reads everything from memoirs and meditation guides to deep dives on autonomous weapons.

There's one kind of book that he thinks is going out of style, though: Textbooks, he writes in his and his wife Melinda's 2019 Annual Letter, "are becoming obsolete."

"I read more than my share of textbooks," Gates says. "But it's a pretty limited way to learn something. Even the best text can't figure out which concepts you understand and which ones you need more help with."

Software can be used to create a much more dynamic learning experience, he says.

Gates gives the example of learning algebra. "Instead of just reading a chapter on solving equations, you can look at the text online, watch a super-engaging video that shows you how it's done, and play a game that reinforces the concepts," he writes. "Then you solve a few problems online, and the software creates new quiz questions to zero in on the ideas you're not quite getting."