The American book industry — and, in a sense, the world book industry — got another major player yesterday when it was announced, amidst swriling rumors that Barnes & Noble was going to sell off its Nook division (which would have signaled the beginning of the end of its brick-and-mortar business) that instead, B&N’s Nook business had taken on a major “strategic partner”: Microsoft.

It was another one of those weird moments in our business where you find yourself rooting for a giant corporation that you would have never rooted for before — because it’s just as likely to wreak havoc as any other soulless giant, except in this instance it looks like it might wreak havoc against Amazon. It’s another instance of that damned “enemy of my enemy” mantra we have to mutter a lot in this business nowadays …

Anyway, as a New York Times story by Michael J.de la Merced and Julie Bosman reported, Microsoft will be investing $300 million in B&N’s Nook division in return for a 17.6 percent ownership stake. As the story observes, Nook sales have been growing, but …

The Nook division’ growth has come at enormous financial cost, weighing down on Barnes & Noble’s bottom line …

The investment from a deep-pocketed tech giant will give Barnes & Noble breathing room, allowing the company to continue to spend money developing Nook devices.

“It gives them a much larger, financially stable partner,” said Peter Wahlstrom, a senior analyst with Morningstar Equity Research. “The bricks-and-mortar side of the business is profitable but all that cash goes into investing in digital.”

For Microsoft, it is an opportunity to expand its efforts in the tablet arena, said James L. McQuivey, an analyst with Forrester Research.

“This is a way for Microsoft to have a hand in the physical tablet business without actually being in the physical tablet business,” he said.

The headline for a CNET News report by David Carnoy puts it a bit more concisely: “Microsoft deal allows B&N to go toe-to-toe with Amazon and Apple.”

And Wall Street seemed to agree: A Forbes magazine story by Susan Kalla last night reports B&N stock “skyrocketed … up 50% for the day. After the announcement, the B&N market capitalization jumped to $1.2 billion from $800 million pre-announcement.”

As to how, exactly, the deal will work, Kalla explains:

B&N and Microsoft will form a subsidiary to market e-books and digital devices in the fast-growing market for on-line content. B&N wants to accelerate sales of its NOOK digital reader and its large inventory of electronic copyrights. And, Microsoft wants to maximize distribution of the new Windows 8 operating system, scheduled for a limited release in June.

… The partnership will develop applications for digital content in the U.S. and abroad. It will explore new ways to store content using digital lockers that control downloads of books and videos for later retrieval by different devices.  The companies will cross sell e-books and multimedia at their respective on-line stores.  And, the subsidiary will develop platforms for users to share, collaborate and publish self-made content. So far, Barnes & Noble and Microsoft each has only a minor presence in the international market so upside in sales abroad will be attributed to the joint effort.

In other words, Google has some concerns here, too.

But for the book industry, the most immediate impact will be upon B&N’s ability to continue cutting into Amazon’s market share — something they’d been able to do quite successfully upon the level playing field of agency model pricing. But that growth — B&N controls an estimated 27% percent of the market — will be quickly stymied now that the Department of Justice has allowed Amazon to go back to its drastic discounting. Thus, B&N had to do something.

And for now, what they did seems to have buoyed spirits in the business almost as much as it buoyed B&N’s stock value on Wall Street. As I say, the enemy of my enemy is my friend … at least for today.