For all our talk of being self-contained, realistically, two books that you can't escape having handy are Hacking Linux Exposed and Unix System Administration Handbookor, more generically, a book on security and another on system administration. These two are our favorites (hereafter known as HLE and USAH). Each of these subjects is much larger than can be covered here, and both are crucial and unavoidable (but also fun). Get them, read them, grok them.