I found the following article good - E Q Primer

You might also try the local library. Ours had at least half a dozen good books on sound and mixing that were really helpful.

I also know little about this as self taught and learning a little bit by bit - but this is something of what I have learned.

The first thing is you may add clarity to a mix by panning instruments to different places and treating it as a 3d thing so that they don't fight in space as well as frequency.

Most of what I read suggested cutting rather than boosting was the key. You increase definition of instruments as you cut out the frequencies where they tend to compete - guitar, bass and keyboards and parts of the drumkit all do battle in the same frequency bands and so make each other indistinct and dull. By cutting and boosting a bit it will help to distinguish them - there is a short section about 'Complimentary Equalization' in that article which is what I think you refer to and it gives a couple of examples.

Do you have access to a parametric equaliser? It's worth playing around with one and see what it does. Usually you can hear the difference as you change the settings as you listen - you can often hear clarity occurring right in front of your eyes/ears(?).

An example - I use a Zoom H2 to record live gigs (some with a 6 piece band of sax/acoustic/electric gtrs/bass/drums/vocals) by popping it out front of the band somewhere. Once I download it I usually apply a parametric EQ with the settings at the bottom of this paragraph (I call it chopmud2) and it takes out a lot of the indistinct mud in the recording and seems to help. It was based on trial and error with my ears but seems to work to my ears - if I turn the effect off it resumes its former muddy sound. Why it works or whether it is a good or bad thing or 'right' I don't know!

There is a 10db CUT at a central frequency of 240hz with a Q of 3 (band type Peak).
There is a 5db boost at 2875hz with a Q of 3 (band type Peak).
There is a 8db boost at 9000hz with a Q of 3 (band type High Shelf).

Someone will now probably tell me that's all wrong but it works for me.