For instance, I still only know some of the basics of how chords work, which seems to be the key to everything when it comes to playing bass. I know that chords are made up of notes from the scale (ex: major chords are made from the root, major third and perfect fifth). I also know about seventh/major seventh/minor seventh, minor, augmented, diminished and suspended chords.
I also know some basics about keys and chord progressions.
One thing I knew (or, at least, glanced over at one point) was that it's not uncommon to play the flatted 7th with the V chord. What I didn't know is that it's also not uncommon to play 7th chords with the I and IV, but the major 7th as opposed to the flatted. It's this kind of thing I want to learn more about.
Another thing I'd like to learn about is what chords to expect and when in any given key. The problem I have is that people generally give examples of two or three chord progressions. Nearly every song I play has a 4-chord progression for each verse. (ex: a song in the key of B that goes vi, I, V, IV during the verses)
Any ideas for places I can go to find more info?
A good place to start is to learn about the harmonized scale. THIS is where chords come from, and it's important to learn. It's easy too. Just take every other note of the scale and stack them up. Then do the same with the next note of the scale, etc.
For example, using C major...
Write out the scale...
C D E F G A B C
Then, starting at the third note, repeat it above what you just wrote...
E F G A B C D E
C D E F G A B C
And again, starting a third from the last line you wrote to get the triads...
G A B C D E F G
E F G A B C D E
C D E F G A B C
And one more time for the 7th chords...
B C D E F G A B
G A B C D E F G
E F G A B C D E
C D E F G A B C
Now take the time to analyze what each of those stacks are in the triads and the 7th chords. It's wrong to say "... it's just understood that a dominant 7 chord (C7) gets a b7 note." Why? Because there's a reason the V chord has the b7 of the root, and it's in the harmonized scale. LOOK closely at the four-note chord built on the fifth note of the scale. It's got a major third from the root to the third, a minor third from the third to the fifth, and another minor 7 from the fifth to the seventh. That F natural is the b7 of the G note, and that F combined with the B (the third of the chord) gives it the distinctive clash and the unsettled sound that pulls your ear to the I chord. The notes are all from the same scale, that's why the V chord has a b7, not because it's "understood".
Same with the I and IV being major- it's inherent in the notes that you get when you stack thirds in the parent key.
Learn this harmonized scale, it will help you more then twiddling your fingers through all the modes

.
Here's a repeat of something from earlier threads about what order to study theory concepts in...
From an earlier post of mine (with thanks to mambo4 for creating the framework)...
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THEORY PROGRESSION
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Theory can seem like a quagmire to those who are starting out, and it's often difficult to know just how important a particular aspect of it is. I will say that learning how chords are built from scales is the most important aspect of theory. It is far more useful to understand chord construction than to memorize all those "Scales A and B go with chord X" formulas.
I'd say the logical progression learning music theory is this:
1.) Learn the major scale, and how it's constructed
2.) Learn how basic chords are built from the major scale- e.g Major is 1,3,5, minor is 1,b3, 5, etc.
3.) Learn how to harmonize the notes of any diatonic major scale by building chords / stacking thirds.
4.) Learn arppegios/chord tones
5.) Learn to look at common chord progressions as "numerals" (eg, I-IV-V ect) to understand how the chords relate to the song's key.
6.) Learn the Natural Minor scale (a/k/a Aeolian mode) and the dominant scale (a/k/a Mixolydian); And learn how these relate to the major scale (i.e.; its the V and vi mode)
7.) Understand how other 4 modes of the major scale are derived (less important to memorize these other modes at first)
8.) Dive back into modes for more detailed ideas about what "goes" with what chord.
Bass playing is basically a matter of knowing what to play over various chords. It may seem daunting at first, but my practical experience (bass in pop/rock) has been that I mostly use Major, Minor, and Dominant 7 related bassline patterns, usually based on chord tones and pentatonics. Even if you're playing some guitar oriented riff-rock, each riff is going to imply a chord of some kind.
"BUT HOW DO I APPLY THIS THEORY TO MY PLAYING?"
85%+ of the time, you will be going from root note to root note as the chords change. The trick is learning how to do it with a groove and feel that is stylistically appropriate to the song. The best way to reach stylistic understanding is to learn songs you like and pick them apart to see how the bassline relates to the chords. I cannot emphasize this idea enough: The answer to this common question is to LEARN AND ANALYZE BASS LINES BY THE MASTERS. Once you undertand what Jamerson (for example) did with a particular set of changes, these ideas become added to your tool set, to use, change, blend and create your own voice.
I highly recommend Edley's Music Theory for Practical People as well.
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John