The "Oxford Companion to Wine", edited by Jancis Robinson, is the definitive modern reference on wine. Not intended to be read as a book, the entries nevertheless make compelling reading and following the cross-referenced entries can easily consume a pleasurable evening. This "supplement" doesn't live up to the original in terms of quality, comprehensiveness or value. If you were expecting a version of the companion tuned to American wines, you'll be sorely disappointed. On the other hand, if you can't get enough of the original and long for more information on California growers, this isn't a bad start. We can always hope for a revised, expanded, second edition.
For the North American supplement, Jancis Robinson served only as a "consulting editor". She apparently corrected the editor's English usage (see the preface), but she didn't write any of the entries. She did write two throwaway pieces in the beginning of the book on "How Good are North American Wines?" and "Commentators and the Wine Media". There are roughly 60 pages worth of introduction to North American Wine, most of which I did not find deep enough to be particularly informative.
Almost all of the cross-references on vinification, wine-making, cellaring, tasting, defects, grapes, etc. are in the "Oxford Companion", making it essentially impossible to use the North American guide alone.
Compared to the "Oxford Companion", the entries are relatively breezy. The font is larger, the margins are wider, and the book is much shorter. Like the "Oxford Companion", the maps are truly horrendous; you'll remember them from coloring assignments in grade school. Invest in Hugh Johnson's and Jancis Robinson's wonderful new "World Atlas of Wine" for maps. The Atlas's coverage of North American wine styles, grapes and regions isn't half bad, either.