IT books are ephemeral by their very nature. My personal experience is that they'll be relevant for 1-3 years, depending on the subject. Also remember that it can take around a year to get a book written and published, so parts of it will likely be dated by the time it hits the street.

I usually buy a book with the intention of giving it one solid read-through. After that, I should be familiar enough with the subject to just google any piece of information I'm looking for, as opposed to paging back through the book.

I think you hit the nail on the head though. If a book focuses on higher level procedures and concepts, it'll have a much longer life than a book that focuses on narrow, highly technical content.

Although, I don't understand your "fear" and "worrying." Most of us are in this field because we're drawn to the rapid change and opportunity to regularly learn new things. It's only natural that related educational material will quickly fall by the wayside. 

Look for good deals on used books at half.com or Amazon. I've bought many where the cost of shipping was higher than the book.

There is also an abundance of free resources, such as SecurityTube, the papers on ExploitDB, Metasploit Unleashed, etc. Be sure to check out the vendors too. Google something like cisco security guide for some good information from Cisco. Microsoft has a lot on Technet as well. You may not get the nice narrative format of a book, but the same content is available. You can learn a lot without spending a penny.

If the book refers to "WEP" as "the preferred method of securing a wireless network..."  then it's time to donate to the local library

The same answer as every other question in the industry, it depends

A few months back I bought all three versions of the Hacker's Challenge books used on amazon for under $20 total. The first two editions are ten years old and while a lot of the specific attacks are no longer feasible they've been great for understanding the mindset and methodology aspects which aren't often covered.

I've got programming reference books written 3 years ago that I'll never touch again.

I've never come across that one before, nice.

Books date at different speeds depending on the technology and the quality of the writing. Some older books like The Art of UNIX Programming are well worth reading despite its age. Java in a nutshell for JDK 1.3 should probably remain on the shelf. You might want to check the reviews in Amazon or aska round on the forum if you want to know about a specific title.

Regards
Jimbob