| I'm not aware of any specific theory for slap bass versus fingerstyle or with a pick... Or any real difference between theory on guitar, or piano, for that matter. The roles of the instruments are different in a mix, but music theory is music theory.
As far as reading music, there are soo many books about theory that it's hard to know where to start. Check out the Mel Bay catalog, and teoria.com, for some good articles and free Flash-based interactive exercises.
As far as knowing what notes are in which keys, that's where the Circle of Fifths comes in - pretty basic-level music theory. There are different scales, too, and of course the modes that go with those, so it gets pretty tricky, but it's easy once you have it memorized. You really only need memorize the basic patterns, and then you can figure out which notes are in which keys from there. For example, a major scale follows the pattern (in steps aka tones): whole-whole-half-whole-whole-whole-half. There are twelve notes total (C, C#, D, D#, E, F, F#, G, G#, A, A#, and B). So, if you're in the key of C, a C-major scale reads C, D, E, F, G, A, B (and then the octave, C). There are no notes between E and F, and similarly between B and C, so the pattern creates that scale. The minor scale pattern is related - you start on the A and end on the G (or play the octave, A, again). So, if you start on the sixth spot in the pattern above, you get whole-whole-half-whole-whole-whole-half (A, B, C, D, E, F, G, and the octave A).
There are exceptions, but that should get you started. Learn some diatonic scale fingering patterns, and learn the names of all the notes on the neck, and where those appear on a grand staff. Teoria.com can help you with that.
Hope this helps! Feel free to ask any questions if there's anything that doesn't make sense, or if you'd like me or anybody else here to expand on something.
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"Mama" Dave Muscato
( www.MamaDave.com)
Ristola 6er/MTD Artist 5er/Ibanez 6er fretless/Line 6 Variax 5er
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