Book Spotlight: K&R, The Bible of C
If there is such a thing as a legendary programming book, then the canonical example would be Kernighan and Ritchie's, The C Programming Language. I leaned on it heavily in college while learning C and I have yet to meet anyone who has done much C programming and does not swear by it. Which is perhaps just an indication that I need to meet more people, but rather than face that idea, I thought I'd try to determine what made this book a legend.
C Itself
If you've programmed in C and in almost any other language, you know that C is hard. C gives you amazing control over your program, with few boundaries over what you can and cannot do. C is deliberately not as abstracted as C#. Fewer boundaries and abstractions means that you can write unworkable code very easily in C. So a tutorial style book explaining some of the mystery is bound to be popular.
C is also everywhere. It could be changing now, but it has long been a staple of college programming courses, providing a continuously refreshing set of programming newbies looking for answers. Finally, C is compact. It is possible to cover a lot of it in a 274 page book; for C# and .NET, that might get you a good index.
1988
The 2nd edition of this book came out in 1988, so long (in computer time) before the massive collective programmers' brain dump that is the web. In other words, when this book was in its heyday, you couldn't just Google for an answer to your coding woes. Oftentimes K&R was the only and best reference around.
Brevity and Style
As I mentioned before K&R is less than 300 pages long, so you didn't need to buy a seat for it on the plane ride. In this case brevity was coupled with great content and good writing. I wouldn't call K&R a page turner, but for a programming text, it was pretty good. How many 800+ page programming tomes have you seen on your desk or someone else's and wondered why the back 400 or so pages looked so crisp?
Short and well written, focused on a topic prone to tutorials and lacking the competition of the web, K&R was able to vault to a position of prominence. So, if you want to write a legendary computer book, find a language that's tough and without much online reference material and of course, write well and concisely.* Pretty simple, right?
* An example of this approach is Programming Ruby. Before Ruby got so popular with Ruby on Rails, Programming Ruby capitalized on some good writing, lack of English language documentation and a quirky language to become a legend of Ruby programming. Maybe Programming Erlang is next?