[Jtsummers]

Different books, different aims. Elements is suitable for both self-study and a 2-6 week (depending on time and level of students) survey course for high schoolers or college students in non-EE/CMPE degree programs. The material is freely available (for the 1st edition, I assume the 2nd will be as well). The print version is 1/4 the cost of Computer Systems, which seems to be aimed at college students (to be covered over an entire semester) or more experienced programmers for self-study.

The neat thing about Elements is that, despite being shallow in parts, it provides the full map of the subject with projects building on each other. Someone who's gone through it can more easily approach deeper treatments to fill in the details as they know where it's going, which also aids in motivation (removes the "what's the point" question when faced with seemingly esoteric topics).