Whether via search or link from a web forum, every few months I find myself back at the fantastic Sound On Sound article Using Equalisation. And as with Equal Time, the Electronic Musician I linked to before, I get something new from the Sound on Sound piece every time I read it.
The article covers the basics of using an equalizer to shape your sound. But it also describes some of the more subtle psychoacoustic issues that a rise from EQ use:
Another psychoacoustic effect which can be manipulated with EQ is the perception of distance. This is because the air damps high-frequency sounds more than low-frequency ones. If a sound source is very close, this effect is negligible. However, the further a sound has to travel through the air, the more the high frequencies are damped. Therefore, if you roll off a little high end from a sound, it seems further away. This technique is often used to bring a lead vocal to the front of a mix otherwise dominated by backing vocals, for example. The backing vocals are cut a little above 10kHz or so, while the lead vocal is given more energy above this frequency.