"Music Theory for Dummies" is a reasonably survey of music theory. It covers all the rudiments (notation, intervals, scales, key signatures, circle of fifths, triads/chords, terms) and has brief sections on harmony, musical forms and historically important music theorists. Overall, it looks like a good place to start, and it seems well written and not too boring.
However, if you already know notation, scales, key signatures and triads, there won't be much new or of interest to you. This book does not go into harmony or voice leading in any meaningful way. If you are interested in these subjects, you'll have to go to something like "Harmony and Voice Leading" by Aldwell and Schachter - I used this text in my music theory courses at University; it is very expensive ($100+) and is not particularly accessible.
Ian and others, what were you looking to get out of your new book/studies? I have some knowledge about this and would be more than happy to share.