Exercises for the reader are embedded in the text. I strongly recommend
that you work these out carefully. If you run out of patience, you can look
up the answers to most of these exercises in the appendix.
The first five chapters are largely based on a series of lectures that I gave
at the 2005 Summer School in Aperiodic Order at the University of Victoria.
I am grateful to the organizers and participants in this summer school for
their enthusiasm and their moral support. I am also especially grateful
to Margaret Combs for her TEXnical and artistic assistance. This book
is partly based upon work supported by the National Science Foundation
under Grant No. 0701055.