Amazon had mixed results in its Q4 earnings call on Tuesday—which also included some dreary numbers from 2012 as well. But that isn’t warding off investors, who rushed to buy Amazon shares in after-hours trading, bumping shares up more than 9% to $284 at one point.

Investors were drawn by one particular detail: operating income. Considered a key business metric, operating income was up 56% year-over-year to $405 million in the fourth quarter of 2012, compared to $260 million in Q4 2011.

That said, revenue and net income missed analysts’ expectations. Amazon announced $21.27 billion in revenue—an increase of 22%--and net income of $97 million or $0.21 a share—a whopping 45% decrease over last year.

Analysts had been expecting $22 billion in revenue and $0.27 a share in profit.

Amazon’s numbers for the year were even more dismal. While operating income for Q4 was up due to the holidays, it actually decreased in 2012 overall by 22% to $676 million, compared to $862 million in 2011. And while revenue for the year was up 27% to $61.09 billion, the company suffered a net loss of $39 million, or $0.09 a share, compared to a profit of $631 million, or $1.37, in 2011.

Amazon is pegging its Q1 2013 guidance at $15-$16.6 billion in revenue, and -$285 million to $65 million in operating income.

There were some other interesting details, however—like the fact that e-book sales are soaring.

“We’re now seeing the transition we’ve been expecting,” said CEO Jeff Bezos, in a statement. “After 5 years, eBooks is a multi-billion dollar category for us and growing fast – up approximately 70% last year. In contrast, our physical book sales experienced the lowest December growth rate in our 17 years as a book seller, up just 5%. We're excited and very grateful to our customers for their response to Kindle and our ever expanding ecosystem and selection.”

Indeed, last year e-books outsold new release hardbacks for the first time ever.

Additionally, Amazon’s digital media selection now includes 23 million movie titles, TV shows, songs, books, magazines, audiobooks, and apps and games, compared to 19 million in 2011. Throughout the year, Amazon made several licensing agreements with Turner Broadcasting, Warner Bros. Domestic Television Distribution, and A+E Networks to expand its Prime selection to some 36,000 movies and TV episodes. 

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