Programming books are relevant for at best 2 years.
After that, the libraries have changed, the languages have changed, heck, the platform may well have changed.
If I need particularilly arcane knowledge for a project and I can find a book that covers it in quality depth, I will buy it so that I can read it a dozen times in a way that computer reading just does not afford. HOWEVER, many other computer books are just not worth the price.
I love books, and I have a library with fiction down one side and technical references down the other. Sadly, the book on computer graphics from 1985 is probably one of the least relevant reads you will ever have. Happily, the math and physics text books are still mostly relevant, as are the dictionaries and grammer guides (well, ok, grammer guides are really less useful in today's devil may type world..)
That said, computer books are way to darn expensive for their expected useful life time.