Mike Dimin's book IS really good, but I've found it to be frustrating. I've found it frustrating in the way that it has opened up new things to me as a bass player, and I'm getting much more real practice time in now since I'm hearing harmony and chord progressions better. Arrgh! His book is GREAT!
But more on chords...
I find that playing shords isn't so bad, and you can really increase your chord repetoire by learning the inversions to chords. The inversions seem to sound more "open" to my ears than playing a straight "root-third-five" chord.
An example of an inversion (if you're unfamilair) is to start with the "normal" C major chord by playing a "C" on the eighth fret of the "E" string, an "E" on the seventh fret of the "A" string, and an "G" on the "D" string. Perhaps this sounds muddy, but wait! You can play it a little differently...
Play the E and G notes on the same strings, but now play the C on the fifth fret of the "G" string. Sounds different now, doesn't it?
Now play the G on the 10th fret of the "A" string, the C on the 10th fret of the "D" string, and finally the E on the 9th fret of the "G" string. Sounds different again.
All three are the same chord, but the character changes dramatically.
There are other neat things you can do with chords on bass, even in the lower register such as playing an A on the "E" string" an F# on the "D" string, and a c# on the "G" string...
Messing around with different voicings around the neck, and thinking as vertically as you can, letting your imagination (or even boredom) lead you to different places are starting points that you can use to get your chordal knowledge and vocabulary up to a level where YOU create the language that is already in you, waiting to find its voice in your playing.