Imagine the mix as being a pallate for painting. You have the two basic dimensions of up/down and left/right. Then there is the third dimension of color. It's the combination of those three dimensions that a painter works with. I take the same approach with sound design.
LEFT/RIGHT
The left/right dimension on the pallate is easy to translate to audio, that's the same as the left/right positioning in audio.
This is maily set by panning the tracks as desired, but there are also other tricks using signal phase, or delay and stereo FX that can augment this as well.
FRONT/BACK
The up/down on the painting would translate to front/back in audio Imagine the painting is a view of the sound stage from above looking down with the front of the stage at the bottom and the rear of the stage at the top.
There are many ways for placing instruments in the front-to-rear dimension, but the main categories would include:
Volume: The louder the signal the closer, the softer the further away.
Reverb/delay: The wetter the signal (i.e. the more reverb there is) is often interpreted by the brain as the further the distance from the source (the further back.) Also, do a Teoma search (Google gets too much free advertising

) on "the Haas effect."
EQ: Further objects can tend to have duller HF components to them. This is similar to the distance effect in painting where the further objects tend to have a bit less sharpness or focus to them.
COLOR
The color spectrum in painting would translate to the frequency spectrum in audio. This of course is the main jurisdiction of EQ, but other types of signal processing such as compression and saturation can affect this to a degree as well.
The above are all general principles for which one can always find exceptions. But the main thing is to approach the mixing stage with the idea of creating a 3D sound design for the mix. Designing a good balance in all three dimensions will give you an expanded sound stage with plenty of room to work.
As far as books go, try "Modern Recording Techniques" by Huber & Runstein and "The Mixing Engineer's Handbook" by Bobby Owsinski.
G.