The Design of the Unix Operating System is one of the best computing books I have read in a long time.
It is clear and easy to understand. (It’s also very cheap if you buy a second hand edition). But it is more than that too: as soon as you begin reading it you realise that it has been fundamental in fixing the culture of the Unix/Linux kernel development world. Indeed I wish I read it long before now as it would have helped me get a better fix on both Linux’s antecedents and on the mental world of the average kernel hacker.
That said it is old and somebody ought to rewrite it for Linux – maybe I will be able to do that some day.