As a tradition, no. Old books like this are often read in a management context. For example, at my local book shop, The Book of Five Rings is in the Business section[1]. The Art of War by Sun Tzu[2] can be applied to any context you want and it's usually still good advice. Re: The Art of War[3], wikipedia says:
> There are business books applying its lessons to office politics and corporate strategy. Many Japanese companies make the book required reading for their key executives. The book is also popular among Western business management, who have turned to it for inspiration and advice on how to succeed in competitive business situations. It has also been applied to the field of education.
Here's where I start talking out of my ass: If you think about it, aren't generals the original managers? They had to coordinate the actions of thousands of people. Sometimes hundreds of thousands. They have to delegate to their subordinates because it'd be impossible to micromanage everything. Also, I think it's more than a coincidence that the quote seemed to fit so well. Outsourcing is hiring mercenaries.
For more "generalship as management," you can read another classic, Caesar's Gallic War. One caveat: a lot of modern Classics scholarship would rather take this less as an example of great generalship and more as an example of great propaganda. But either way it's a treatise on how to command, containing if not quite the unvarnished truth, at least Caesar's opinions of what good management/generalship ought to look like, as well as several anti-models.
The core concept in this paragraph is really a basic concept in outsourcing - don't outsource your business's core product, or you will be at a disadvantage when competition comes.
Notice, he doesn't say that you cannot use these extra forces in attacks. All he is saying is that if they are guarding your sleep, you are probably screwed. Similarly, if your core business is being a technology organization, if you chose to outsource that core tech, you are likely screwing yourself. Now, should outsource as much as possible the non-core aspects, either by buying software/services or by hiring consultants, while you let your core team focus on what they do best.
Of course, it's all easier said than done, and the line between core and non-core is often blurred.