The textbook for the course is Computer Systems: A Programmer's Perspective, Third Edition (also known as CS:APP3e) by Randal E. Bryant and David R. O'Hallaron. Pearson, 2015, ISBN 978-0134092669.
The textbook is long, but comprehensive, and past students have found it very useful as reference during the class. Assignments and exams may assume that you have done recommended reading. It is strongly recommended that you obtain a copy of this textbook. It may be purchased at the Harvard Coop, or from one of many online bookstores.
Copies of the textbook are available at both the Gordon McKay library and the Lamont library.