  We present an extensive study of the foreshock and aftershock signatures
accompanying peaks of book sales. The time series of book sales are derived
from the ranking system of Amazon.com. We present two independent ways of
classifying peaks, one based on the acceleration pattern of sales and the other
based on the exponent of the relaxation. They are found to be consistent and
reveal the co-existence of two types of sales peaks: exogenous peaks occur
abruptly and are followed by a power law relaxation, while endogenous sale
peaks occur after a progressively accelerating power law growth followed by an
approximately symmetrical power law relaxation which is slower than for
exogenous peaks. We develop a simple epidemic model of buyers connected within
a network of acquaintances which propagates rumors and opinions on books. The
comparison between the predictions of the model and the empirical data confirms
the validity of the model and suggests in addition that social networks have
evolved to converge very close to criticality (here in the sense of critical
branching processes of opinion spreading). We test in details the evidence for
a power law distribution of book sales and confirm a previous indirect study
suggesting that the fraction of books (density distribution) $P(S)$ of sales S
is a power law P(S) ~ 1/S^{1+mu} with mu close to 2. A short version of this
work appeared in Sornette, D. et al., Phys. Rev. Letts. 93 (22), 228701 (2004).
