This book had its origins in lectures presented at EPFL, Lausanne,
during two separate visits (the most recent being to IRRMA). The author
is most grateful to Professors A. Baldereschi, R. Car, and A.
Quattropani for making these visits possible, and for the splendidly
stimulating environment provided. Professors S. Baroni and R. Resta also
influenced considerably the presentation of material by constructive
help and comments. Most importantly, Chapters 4 and 5 were originally
prepared for a review article by Professor G. Senatore, then at Pavia
and now in Trieste, and myself for Reviews of Modem Physics (1994). In
the 'course of this collaboration, he has taught me a great deal,
especially about quantum Monte Carlo procedures, and Chapter 5 is based
directly on this review article. Also in Chapter 4, my original draft on
Gutzwiller's method has been transformed by his deeper understanding;
again this is reflected directly in Chapter 4; especially in the earlier
sections. In addition to the above background, it is relevant here to
point out that, as a backcloth for the present, largely "state of the
art," account, there are two highly relevant earlier books: The
Many-body Problem in Quantum Mechanics with W.